I liked how naked Mystique actually makes sense for a shapeshifter. When she mimics someone, she also mimics their clothes which are themselves part of her. The alternative is to have her spending half the time knocking people out and nicking their outfits.
Having said all that, I think they cheat with this a few times in FC as I think there are a few time where her clothes change while she's supposed to be wearing actual clothes.
It would be neat if at some point they do give the classic white dress a nod, but I can't say as I miss it. I have to ask though, why a skull belt?