Just because they're commoners doesn't mean that they don't have ranged weapons. Even thrown rocks can be deadly if they're the size of your fist and dozens are being thrown at you. I also don't assume they have good fighting form - it's more of a crush of people trying to overrun the PCs with shear numbers so they don't require 5 foot squares per individual.
I wouldn't call a hundreds-strong force of bowmen a mob. Thrown rocks, sure, why not. But they might very well be defeated easily by low level spells. Spike Growth cast around a PC will make him unable to be engaged in melee even with reach weapons (the 4 HP commoner would die after walking 5 to 10 ft, half the distance necessary to be adjacent to the PC...). Tightly packed, pushing and shoving mob would die by dozens before realizing the deadly spikes are impaling them wholesale. And your mob trying to enter an area of hard spikes and thorny enough to
kill you after a few feet of progress would certainly have the pushed-in try to escape on the side rather than press onward into the shrubbery of doom... stampeding themselves while the characters just stand still. That is, unless the spikes are covered in a (1st level) fog cloud,
from which none of the peasants will emerge alive.
(TBH, I'd rule that after four of five dead bodies on the same square, the body pile towers above the thorns, but it would be against RAW).
Sure, on the next wave the mobsters will want to try to throw rocks and avoid tightly packed formations (because if they are two in a single square packed, like a phalanx unit, a single fireball would kill 88 of them in an obvious, loud, and blatant display of power. Sure, it's possible to keep cohesion after that but they'd need to be motivated to fight to the death).
They'd have better chance trying to mount an ambush or harass the PCs with guerrilla warfare rather than rushing at them, zombie-like, unless they have some situational advantage.
There's a reason Frankenstein's monster always ran from the torch and pitchfork wielding mob.
Yes, and its "a definite lack of machine-gun". Warriors PCs would be useless against a mob, it's spellcasters that would make the fight one-sided (unless ambushed).