Maybe its because they're quick to judge the person's character, or maybe they're just jealous. There are always going to be small minded humans out there who make fun of others, don't let it get to you.
i have no idea, but like every girl in my school wears them, but its always the stupid ones. and once this girl walks up to me (i moved to this small town from nyc, usa), and she is like why dont you wear aeropostale. and im like why should i? her responce?- but you were raised in nyc, its like where aeropostale is from. this is why they make fun of them :/
its because its so popular, so many people shop there that its become over used and everyone wears it and people make fun of them because they see it on every one! maybe if some one wears something different this cycle will break.
People who make fun of anything are usually insecure about themselves, probably in the area of what they are making fun of, and they want to distract attention from what they consider their own faults.