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.
Alibaba - Magi: The Labyrinth of Magic
H/A Elsa - Frozen
Yunan - Magi: The Labyrinth of Magic
Mephisto Pheles - Ao No Exorcist