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.