1. it's one of their most popular songs
2. it's the most thematically appropriate song. creep is about a man trying to get the attention of a woman they're attracted to, but in the end, lacking the self confidence to face her.
this is EXACTLY the correct song for this scene, Koga by this point sees aya as someone she isn't: confident, gorgeous, popular, and has a lot of friends.
meanwhile koga constantly has issues with self worth and doubt. the chorus verse "what the hell am i doin here? i don't belong here", is pretty much describing koga considering at this stage in the manga, she lacks the self confidence at school that she has at the record shop.
koga singing this song (whilst not confirmed anywhere official to my knowledge) fits the most. she's calling out to aya, telling her SHE'S the special one, whereas she feels as though she's the role of the creep. the one that doesn't belong in her world.
regardless, it's a call to aya, and she ends up at the end confessing she wants to have a TRUE friendship with aya.