Firstly, in my books any decent human being does not judge a person by the clothes or lack thereof on their back. That applies to everyone, everywhere, in any setting. More specifically to cosplay, however, we're all cosplayers because we're nerds or geeks. Until such a time as there is an international standard for cosplay, no-one can be accused of not cosplaying "correctly". You simply cannot tell someone else how to be a fan of the things they love or how to express that fandom amongst like-minded folks. Any attempt to do so makes you the one that is damaging the community, not them.
I've said it many times before and I'm considering getting it tattooed on my face: Let's coexist. Make cosplay what it should be: A safe, diverse, happy place for everybody. Push-up bra or no push-up bra, we're all nerds here.