I guess everyone has different reasons for why they're doing it, but why are so many people saying "oh everyone just assumes you're a big nerd"?
1: Isn't that kind of true? I mean, isn't 'nerd' by definition someone who is somewhat obsessed with a particular IP or work of fiction? Wouldn't spending hours upon hours and/or lots of money to make a costume looking like a character from that be considered a 'nerdy' thing to do?
2: I thought being perceived as a nerd would a good thing, since it dispels the idea of people dressing up as characters they know nothing about?
Anyway, I honestly don't have an opinion one way or the other regarding Cosplayers, and the only misconception I've really known is that some people see it more as an opportunity to get noticed for modeling jobs than displaying their love for particular character(s).