I think it's ok if they cosplay whoever they want as long as it looks good one them and it's made well. I fully agree that it should quality of the resembelence and how well the costume is made over what the person looks likes. some people are retarted and can't get past that, but no matter who you are you have to be thick skinned to cosplay anyways.
I agree that there aren't only thin/skinny people in anime, but most of the main charaters are, so it's not right to be that limited. I've seen many things modified that looked really good on cosplayers that didn't have thin body type.
I'm also getting tired of all these "sexy" versions of stuff and just feel it's kinda lame and not really cosplay anymore unless a lot of thought is taken into make it resemble a character.