If people are standing around waiting to take photos with cosplayer Z, and you'd like to get one as well, Go stand there. If Z looks at you, ask "may I?" or at least hold up your camera with an inquisitive look.

Across the aisles - legally yes, in public spaces you can't be prevented from taking photos of other people.
That said, said cosplayer has probably put a lot of work into their outfit and that probably includes working out poses that present the character and their body in the most flattering light. It's respectful to allow them to choose how they wil be photographed.