I don't know if your comment was related to Mandy and Billy's relationship as a couple or just being great friends.
Many people see them as just great friends, but to be honest, I see them as a duo that has that little potential to be a couple. Because also, what you wrote makes sense, how Mandy can sway Billy who is really a very extroverted boy and who uses 1% of his brain to do anything. Since she can treat and sway Billy being the terribly terrible boy, why not treat other characters the same way? So if you stop and think about it, Mandy has her own reasons to always be with Billy, such as making him the best person to be by her side, because in Billy's head Mandy has become the person he can always turn to, and she reflects that, like making the reaper do Billy's bidding immediately. Of course, she can't accept doing everything for Billy, because some things are kind of "absurd" for her to do.
But something that made me more confident was the episode in which Jeff the spider simply realized that Mandy was his "mother", and even though Billy didn't want to have the responsibility of being the father of a spider, he sought Mandy's help, asking her to become his wife, to pass all that weight on to her to solve. But she really didn't mind Jeff treating her as if she were his mother, and this was already stated twice in the cartoon, that Jeff didn't forget about her. Mandy herself wouldn't accept that she would have a romantic relationship with anyone, especially with Billy, but Billy doesn't mind saying that he likes Mandy, much less having interactions with her. Just like he does everything for her, she reciprocates in her own way.