I'm going to sign my ex up for a sugar daddy site without her knowing
That makes me feel rather uneasy. Is there a reason why you want to sign her up?
No, I just want to mess with her for being a bad girlfriend
You shouldn't do pranks on your ex. It makes you look childish and she will justify her reasons for being a bad girlfriend.
Who cares? As long as I don't get caught.
It is not about getting caught it is about what is right or wrong. Think about how you would feel if someone did this to you.