Reddit's rules on comments under cosplay posts should be the internet's rules for comments under cosplay posts.
I obviously do agree with the first part of the rule (I mean, come on), but not the second part about "the person in it". Let's take as a example a very popular character to Cosplay, Capcom's Morrigan Aensland. The very basis of the character is that she looks hot , she is a Succubus after all, so if the person wearing the costume does look hot, then it adds to the quality of the cosplay. The cosplayer her/himself is as an integral part of the cosplay as the costume, and good discussion about the craft should allow comments about the person wearing the costume.