She participated a superhero costume contest in that costume. Bart was also pushed into the contest, as they thought he was a contestant instead of the real Bartman. And the most embarrassing thing is that Lisa won the contest, and Bart, despite being the "original" one, only got free tickets for a taco restaurant.
I don't right now remember which comic it was in, but here you have some images featuring her:
Fan-made art of Bartman and Bartgirl: [link] Art based on the comic book: [link] And this is what she looked like when she really made her appearance: [link]