There will always be someone that will snob other people's cosplays. Unfortunately with hardcore fans, they sometimes get too carried away and forget that cosplayers have feelings too. Unless they are just jerks.
But anyway...you should just ignore them, cosplay who you like, and have fun.
I don't really like the whole "You should cosplay based on your race & body type" line of thought. I don't think it is very fair as certain races and body types are very rare. I could probably count, on my hand, the number of plus sized female anime characters. However, as someone who's a little chubby (not plus sized & I hope to never be...), I've noticed that people tend to focus on the skinnier ones more. But that won't stop me from cosplaying who I like, even if I'm not no size 6.
I also have friends, of different race than said character, that have pulled off better versions that those of character's "supposed" race...
@Anix: I believe we might have the same friend. XD; Either that or you are friends with my friend's cosplay doppleganger.
@domofudge: You shouldn't let jerks like that keep you from cosplaying who you want. You need to tell people like that to back off and if they continue to harrass you, I believe you can report them to con staff. Especially if they destroyed your wig (if I'm understanding you right?) Besides, no one should ever touch you or anything on you without permission.