As it's just a title, I wouldn't say one or the other makes more
sense but sure. Both works. Just saying that it would be more common to have it as a girlfriend-related. If it IS a song abt them working hard etc then obviously it would be "I want to be her." makes more sense. But if it's a typical idol song, it would make more sense to have it translated as "I want to be your girlfriend." Which we will see when the song is performed.
Sorry for thinking as these kids as idols doing idol songs, I'm obviously way wrong.