Marco is clueless. It's part of his personality. But he's also bright.
Marco strikes me as the kind of person who weaponizes ignorance to deescalate tense situations. it's something that I am really good at doing
When he sees Tom and Star together, it's better for the situation to pretend like there isn't anything weird going on. When he gets his friendship rejected from Star it's easier to pretend like it's all from left field. It's really hard for Marco to talk about the elephant in the room, so he pretends he can't even see it, and most people think he's just totally clueless.
The faulter in the writing, I think, is that we never get a moment alone with Marco, where he's not putting on a performance to make sure the situation is smooth. If we got even one cutaway shot at the end of some episode with Marco looking at a photo of Star and just sighing or something, I think it would have made it better.
Because when Marco realizes he can't be with Star, it's not like how Star's heart broke when she saw Marco and Jackie kissing. It's Marco coming to the realization that he can't keep ignoring the way that he feels. Star's feeling was "Oh shit, I really can't be the girl for him and I thought this didn't hurt but it does," and Marco's feeling was more "I have to stop lying to myself and realize that I can't keep pretending like everything is the same."