The worst thing about it is that she wears that shit to get Robin's attention and is completely fine wearing it despite her personality changing to her "normal girl" act she had in Awakening and the cold itself.
Besides, the difference between this and her regular attire is the color and the lack of the see-through bodysuit. All the other characters use the Christmas costumes for their own reasons anyway: Lissa wears hers to look cuter than usual, Chrom wears his because Lissa forced him to and Robin does to use his tactical knowledge to create plans for effective present delivery.
The worst thing about it is that they do this over the fact that Christmas Tharja, along with the other three Christmas units are considered Armored units. Hell, they're all top tier armored units with Tharja being a monster out of the box if you wanna get into gameplay specifics.