It isn't out of character though.
Keep in mind she has long been established, since her time as a X-men, to be conscious of her appearance and having a desire to be treated as normal (sadly the Alan Davis characterization reduced her to just that), which naturaly clashed with her outward disregard towards "pretty people's weaknesses" e.g. bothering about appearance and her proud to be a freak mentality.
A contradiction which has completented her overall mixed personality.
So here she got invited to this big Gala, where all the "pretty" and important people will be and the dress code is "be what you want to be, as long as you look stylish". Meaning she finaly has an excuse to wear something she might have wanted to wear since she was a rags wearing freak kid sitting around in a dark tunnel reading cheesy romance novels that got thrown away or forgotten on subway stations and nobody is socialy allowed to question her choice of cloth or appearance.
Now if she was assigned to be a guard at the gala or expected to dress as part of a team, hence in a quasi on the job basis ala. X-force, i could totaly see her being in pants instead. But here she can wear something she isn't expected to otherwise, hence a dress makes sense and i think the artist makes it work.