If everybody had fun, ok, great, there is no wrong way to play!
However, the DM seemed surprised that they had no problem with that fight.
The fact is, from a purely mechanical perspective, the DM helped them way too much!
Mistake #1 - Assuming that goblins are incredibly stupid.
Goblins are cunning little bastards.
They can take full advantage of terrain just as good as any PC.
They should take appropriate tactics, too.
Not rush to their slaughter along some kind of predetermined program.
They know where, and how, to make a proper ambush. Not a fake easily foiled one!
Goblins are notoriously cowardly. So they know then to run, too.
Mistake #2 - Ignoring the module
That encounter specifically says "the goblins can't be surprised".
In other words: the goblins spot the PCs.
Of course, if you radically change the encounter from something (here, an ambush where the PCs get almost shafted)
to something else entirely (suddenly, it is the goblins that are the ones getting almost shafted),
then it is obvious that you are also greatly changing the difficulty.
Mistake #3 - Not thinking about symmetrical situations.
Passive checks are to simplify things, a mere tool for the DM.
They are not to give almost-guaranteed-auto-successes to the PCs.
If the roles had been reversed, with players completely unable to spot foes before
it is too late DESPITE setting up a nice ambush, can you imagine their reaction?
They would say the DM made unfair rulings. Or worse.
Well, here it was exactly the opposite: the situation was quite unfair for the goblins.
Avoid giving that kind of overwhelming advantage to the PCs.
In D&D 5e, sight is not a tiny pinpointing laser beam, is but more or less a 360 degrees thing.
There is no such thing as "attacking from the back". Perception works all the time, in all directions.
If you add "it is coming from the back and for some weird reason the goblins really focus their attention forward and only forward",
then at worst they should get some kind of Disadvantage, not an almost guaranteed automatic failure.
However, if that "reason" was to occur, then you should also compensate by making the goblins get Advantage for the "forward" direction, no?
Fair's fair, after all!
However, the goblins don't need to get Advantage for "forward" because that road is open and everything on it is thus very easily visible.
So why would they decide to wear some kind of virtual horse blinders? They wouldn't.
Goblins aren't stupid. So because they don't need to super-fully-concentrate on looking ONLY forward,
they do get their normal sight for looking "behind", too. As per the normal rules.
Well, actually, compensating some just-made-up-penalty when looking "behind" with a benefit when "concentrating forward" is a bad
idea anyway: do it, then players will start invoking that benefit in every situation that they can relatively safely "concentrate looking forward".
Heck, the frotn PCs will concentrate forward, the middle PCs concentrate sideways, and the back PCs concentrate backwards, turning the
entire party into super-spotters that always have Perception Advantage. Not good. Always think about the "symmetry of potential situations".
Passive checks should be used more for when a creature has its attention reduced because it is already busy doing other stuff.
But these goblins were on the lookout, "spending" their turns doing Perception. They should have benefited from Active Checks.
In fact, I would have simply not asked for rolls.
PC Passive Perception vs Goblin Stealth secret roll.
Mistake #4 - Making too many rolls
Avoid making a check for each goblin. Roll only once.
Similarly, when it is the PCs that are making rolls, determine if each PC benefit from success or suffer on failure 100% individually.
In any situation where the fate of the entire group can be affected by a single succeeded or failed roll,
then either allow only a single roll TOTAL, or make it a Group Check instead.
And don't allow rerolls: assume they get the same thing afterwards.
Otherwise, in, for example, a "Perception vs Stealth" situation,
allowing multiple rolls becomes basically just making sure that Stealth never works.
Assume the goblins properly set up their ambush.
Or in a "open lock" situation, you are basically making sure that the door is NOT an obstacle at all.
"Try again until you eventually succeed" doesn't make for very intense rolls.
When you know you have only once chance, though, that roll suddenly becomes important!
When another player says "I try too", don't allow a roll, use the 1st rolls result.
Again, avoiding the "multiple rolls screw up guaranteeing auto-failure and auto-success problems."
Mistake #5 - Not using proper terrain and obscurement.
Let's assume a perfectly straight 5 feet wide trail road, with 15 feet of open
grassy space on each side, and then the forest further along the sides.
A goblin 30 feet from the road, thus 15 feet deep inside the forest, counts as being lightly obscured, right?
Right! This means that PCs should have had Disadvantage on their Perception checks to spot them!
Don't make them roll every round until they are sure to finally succeed, either. They get only one roll: the one that counts.
Either they spotted the goblin just in time, or they failed.
The same goblin is now attacked by a ranger standing on the road, but 120 feet away, thus firing along a "diagonal line".
Well now, that goblin, from the shooting perspective of the ranger, is actually "60 feet deep" inside the forest, not 15 feet.
(from the perfect straight road, along line of sight: about 60 feet of open grass, then about 60 feet of forested area).
At some point of "forest depth", you look through more and more and more foliage, so it starts hiding everything more and more completely.
So that very probably counts as if the goblin was being heavily obscured by the forest, instead of lightly obscured!
You aren't forced to make that forest a nice thin-foliage area. The goblins probably chose the spot with the thickest foliage anyway.
So yeah, 60 feet away, no more.
Now, the battle map shows a VERY windy road, not a perfectly straight road.
Given that the goblins are not so stupid, they must have chosen their ambush spot very cunningly.
This means that any PC "outside" the battle map probably gets an even worse quality of "line of sight",
compared to the "'ideal" situation of a perfectly straight road.
In other words: Don't give the PCs a free lunch unless it is on purpose.
In other words: Don't allow the PCs to spot enemies from too far away.
On a perfect Perception roll, they detect the goblins at the maximum distance you say starts counting as "heavily obscured".
Let's say, 60 feet of forested area.
On a "barely enough" perception roll, they could spot from say half that distance.
Only 30 feet. Quite near the goblins, then! Basically, they are in the middle of the battle map, just not surprised.
Mistake #4 - Not using basic goblin capabilities and natural tactics
OK, so the rogue backtracks, and the ranger starts shooting. This makes his position known.
Splitting the PC party should be extremely dangerous for whoever is alone at the forefront.
goblins aren't stupid. They know very well that their shortbows shoot very badly at long range.
So they will definitely attempt to close in first.
One of 3 things should have happened:
- If ranger is very far away (which shouldn't even be possible on a winding trail in a forested area):
Goblins should have simply fallen back into the forest (i.e. to become heavily obscured),
then circle around in order to come back slowly to attack from another angle.
- If ranger is a little bit far, Hide to manoeuver to come closer. Instead of getting useless "Disavantaged" attacks,
the goblins would seem to dart from bush to bush, closing in.
The melee goblins stick with the ranged ones. goblins are cowardly, so they will attack only
if they can do so en masse, not one after the other or without the support of their archers.
- If ranger is close enough, NOW they gang up on him!
In any case, the Goblins should ALWAYS use their Cunning action to Dash or preferably Hide.
At least give them an easy Intelligence check to avoid choosing poor tactics.
Another thing is don't make goblins super-specialists. Let them adapt very quickly to the battle.
The archers can switch to melee as needed. The meele guys don't have bows but can still shoot a javelin or two.
That way they don't "waste" their turns.
The biggest mistake is allowing the party to have spotted their enemies from very far,
and letting the battle be dealt with at such a range that the goblins had Disadvantage,
for several rounds. Such a situation is akin to giving several free attacks for ranged players.
Goblins are _THE_ ambush specialists. they are cunning, fast, and cowardly.
If an ambush seems likely to fail, they are quick to withdraw and try again later,
when the conditions are more favorable. Like when the party is sleeping.
Or when the party starts investigating the goblin lair.
So, it is no wonder the fight was too easy. Nothing to be surprised about here!