This seems like a silly question... until you think about it.
So Mystique can transform into anyone. And that includes clothes. And every time we've seen her use her power, she's always gone from naked Mystique to whatever fully-clothed person she's impersonating. But clothes don't just come out of nowhere! Matter doesn't work that way. So either her body is changing shape and expanding to include clothes, or the entire thing is an illusion.
Which begs the question: Is she just walking around NAKED all the time?
That's Mystique cringing because that sandwich was in one of her pockets... and she honestly has no idea where those pockets go.
Seriously though, am I the only one who's thought about this?