I'm not saying he hasn't made any progress but he's been really overshadowed by other characters. Even though he's gotten the leading role in more episodes since Season 1, they're just consistently boring or bad without much real consequence. Most of his episodes in Season 2 were about fucking karate. Nothing he did in the movie changed anything, even Buff Frog accomplished more.
Like yeah the kid's definitely become more brave and confident thanks to being around Star, but that was true by the end of Season 1. He manned up and spilled his guts to Jackie, even if it took being controlled by a demon curse and fifty episodes of Star being his wingman to do it. But he still hasn't actually gotten to know her like he said he wanted to, and Star even has to push him to actively date his own girlfriend.>He even has a personal villain now
Heinous is a joke villain and miles away from being as interesting as Ludo, Toffee or Eclipsa.
The two times he's interacted with Toffee were far more personal and impactful than playing dress up, and he's only dealt with Heinous twice too.
I'm sure she'll get much more developed later down the line but until that happens there's just that potential. And even then she's clearly tied to the Butterflys and even Toffee through Rasticore, so who knows if she'll stay just "Marco's villain."
I fully understand that the show is barely halfway through its runtime, and I'm sure Marco will get his turn sooner than later. He's had it too good for too long and shit's going to blow up in his face in Season 3.
But until that happens he's just pretty nice, cool, respectable kid who's still not all that compelling compared to the great cast around him.