Translation:Your dress is prettier than mine.
"your skirt is more beautiful than mine" was rejected but I think it's fine
"Your skirt is prettier than mine" also correct, and skirt was accepted as correct translation for 裙子 on another answer
It's very difficult to rebuild the sentence with our occidental way of thinking. Personnaly - even if I know I shouldn't do that -, I'd translate it like : " Your dress compared to mine is more beautiful ".