Marvel Comics gives its Spider-Girl costume a skirt because all girls must wear skirts!
Spider-Girl is the Spider-Spawn of Spider-Man and his Spider-Love, Mary Jane Watson. Spider-Girl, much like her father, wears an insanely tight body suit while fighting crime, coasting between buildings, doing upside-down kissing, etc.
But whatever corporate entity signs off on Marvel Comics' costume licenses thinks Spider-Girl should wear a skirt. Because she is Spider-Girlnot Spider-Boy or Spider-Man, obviously.
The other unfortunate thing about this costume is that, unlike Spider-Man costumes made for children and adults, the Spider-Girl getup does not come with those grotesque built-in muscles that are so weird and great! This, despite the fact that Spider-Girl would have to be just as ripped as Spider-Man, or else she wouldn't be able to do "what a spider can" all-day, every day.
The Spider-Skirt also comes in pink, which is a great color, but not Spider-Girl's color.