is it a big deal? Probably not. But if you're going to disregard the cosplayer anyway, why make a pretense of having manners.
I've had people tell me no for photos, and while sometimes a reason doesn't seem obvious, if a person says no, you should respect that. I've said no a few times, usually because I was running late for something.
Upcoming Costumes for 2015
Alibaba - Magi: The Labyrinth of Magic / Sasha Braus - Attack on Titan / Howl - Howl's Moving Castle / Molly Grue - The Last Unicorn / Cassandra Cillian - The Librarians