It aint that deep. Her costume was ugly. That wasnt simply bc it was a dress. The dishwashing gloves, horrible mask and clunky boots all contributed to it being a very unflattering look for her. And you are the one comparing hers to males when there are males with dated ugly costumes that get their own complaints, like Colossus. Mystique is wearing a dress but no one complains about it bc it isnt ugly and has aged well in a way that the MG dress hasnt
And Jean isnt wearing an evening gown. Its a pantsuit which is on brand for her