You're not wrong. Marco went from having his own harem to being the show's punching bag. The gift he got, for helping Mewni with Toffee, turned out to be the king's meat wrap. He broke up with Jackie. He killed his relationship with Heckapoo, to protect Star. He went to the beach with Star, whilst Star was dating Tom and he got sad about it. Star doesn't appreciate the sacrifices he's made for her, nor does she even ask him how he's doing. Heckapoo replaces Marco with bird guy and I'm sure I've forgotten a ton of stuff.
Again, you're not wrong. It didn't feel like it, watching on a weekly basis. But, looking back, season 3 was just really toxic.