No, it's not.
Warlock 2/ Sorcerer 3 vs Sorcerer (or Fiend Lock) 5
The straight PC is tossing out fireballs, fly and counterspell. Bladelocks are swinging twice witht their weapons.
Sure, it is. At this tier, the Sorlock is basically like a ranged Fighter who gave up hit points for extra Action Surges and pushes bad guys around for free. Every additional level at this point just grants him the ability to Action Surge more often. Once he hits 5th level, he basically never needs to spend a major resource on single-target damage ever again. Now, it's true at this point, a full-classed Sorc hands-down beats the Sorlock at AoE, because Fireball is a big deal (btw, a 5th-level sorc learns 1 3rd-level spell, not 3). We're not at the tier where it's obvious that the Sorlock is blatantly overpowered, just something of an outlier when it comes to multiclassing. The DM isn't pissed off to the point of house-ruling
yet.
Since you brought up the Bladelock, the Hexblade Patron is another example of badly designed, front-loaded power creep tied to the Warlock class. Sorlocks and Bardlocks were bad enough, but now it's become quite popular for Paladins to 1-dip and 2-dip Warlock now, since using Cha for your weapon attacks, gaining a devastating ranged attack, and the Hexblade's Curse makes a Bladelock 2-dip possibly the most overpowered dip in the game.
Now that I think of it, Hexblade is probably the best option for a Sorlock, too.
XGtE wasn't out when I banned the combo (
this is wrong, it was a Storm Sorcerer), but since the Bladelock's proficiencies are tied to patron, not class, and therefore not in any way restricted by multiclassing, the Sorlock can pretty easily get 19 AC with only 14 DEX, since you can cast EB while carrying a shield. He also has the Shield spell now.
It's like the 3.5 Theurge. Looks great on paper, and then you play it and realize just how sucky being 3 levels behind in spell casting really is.
No, it's more like the 5e Sorlock. Sounds weak on paper, then in practice, when you realize their "cantrip" is more like martial character's main attack, and therefore they stop using low level slots for pretty anything other than Hex and Quickened Spell, it's pretty annoying.