Reason she calls herself a bad girl is because the entire premise of this manga only happened because of her cunning and manipulation behind the scenes.Not that I'd call her a bad girl, but she is a bit manipulative.
She feels a bit guilty about all the strings she pulled to get what she wanted but wants it so bad she does it anyways. Hence, bad girl.
I don't think itll matter since he clearly loves her back, but imagine if he didnt like her. The manga would be a lot darker with her actions being basically the same.