I personally have no problem with overweight people doing cosplays of skinny people. For me, I tend to look at the quality of the costume itself, if you're the right size for the character too, then it's a bonus, but otherwise it really, really doesn't matter.
However, it kind of irks me a bit when I see people not dressing to their body type. Okay, I know it's only a costume and everything, but if you were overweight (as I am tbh), I'd assume that in day-to-day life, you'd dress in a way which covers and disguises your bad bits, and emphasizes the fact that you have amazing boobs/legs/bum or whatever.
Why then, when it comes to doing a costume, would you throw this all out of the window and cosplay as something which shows off all the 'bad areas' and makes you look worse? Especially when it's fully expected that everyone will be taking pictures, which will probably end up on the internet (not /cgl/ in particular, but also Facebook, DeviantArt and Flickr etc.)
I guess another point is how the costume makes you feel though. If you're happy and confident enough to do a costume, go for it! Seriously, you'll have fun, and the vast majority of people will not give a flying crap about what you look like. If you don't feel completely awesome, it could ruin your day.
e.g. The costume I hate most, is my original Miku, not only because it was poorly made, but it made my arms look awful. On the day of the actual con, I felt unbelievably self-conscious and spent most of the time trying to hide my flab. I actually just couldn't wait to get it off at the end of the day, and I kind of regret doing it...
However, my absolute FAVOURITE costume, is Sandplay version of Miku. The dress itself is a lot more flattering, the arms are covered and it shows off my legs, which I guess are the probably the best part of my body. I felt confident, and had an awesome time which probably came across with how I interacted with everyone too.
Gah, I know I probably sound like a bitch, but this is just my 2 cents... Sorry if I pissed anyone off D: And sorry about the essay of a post ><
Taokaka; Blaz Blue
Jennyanydots (Gumbie Coat); Cats The Musical