Marco was great during season 1-2, he changed from the kid focused in studies that nobody knows (maybe janna, his 2 pals, and one teacher), to be someone. Gets his dreamgirl, gets pussy from Hekapoo and Jackie (2 best girls), and get his personal scissors, that for human standards (his age specially), are a feat of strenght and resilence.
Then he went full retard mode, like literally a crayon eater level... he ditches everyone, just to go Mewni and be a nobody that no one respects NOT EVEN THE PEASANTS. and he continue to degrade himself more and more just for star...
welp... that's what happens when the MC is a piece of crap and they need to nerf everyone else around her to make her look like someone who's getting better in the "character development" aspect.
Oh.... and he was literally retconned to go and say that he was "in love" with star since season 1.