She's saying something about Taiga's fly being down, but I'm not confident enough to put in a translation. It's something like, "Did you forget to pull up your zipper?"

I think it's supposed to be a pun. She's asking him to zip up his dress, but then it reveals that something else was zipped down as well.

I know the JoJo reference joke has been driven to the ground some years ago, but can't help to notice one here.