Television

He’s basically leveling up with every fight

Key point here is that he lost decisively to the pony tail, and was wrecked by Anissa. He momentarily held his own against a relatively weak, and past her prime viltrumite, and didn't stand a chance against an elite like Anissa. @John_McMurray also makes a good point about Mark's abilities.

However, I will concede that this is a piece of pop culture, and you shouldn't have to be a big fan of the comics in order to understand why the cartoon isn't inconsistent. THAT is bad writing.

That's actually a key plot point. Mark comes back more durable every time he gets his ass put in hospital. What doesn't kill him literally makes him stronger. Because he is constantly losing to galactic level threats instead of working with them, eventually he becomes the strongest one out there. (Also that scene meant to imply he had to quit acting like it was a boxing match and get vicious if he wanted to live)