I'd argue Marco is much more the main character than Star is, despite the show's title. He's the everyman stand-in for the audience, the straight man to the host of outlandish dimensional antics, and is more often than not the one actually solving the problems.
Star and Mewni weirdness act as agents of the plot, setting up whatever whacky hijinks occur within the episode, while Marco then acts to bring about resolution to said problems.
Marco is Odysseus, Star and the Mewmans are the gods of Olympus and every fantastical beast and obstacle standing between our hero and safety.
Marco is the one who actually stands to get hurt and lose something in each episode, whether it's his life, home, family or sanity between all the extra-dimensional bullshit.
I like Star when she's fully committed to the multiverse weirdness, and when the only lives being threatened are ones that would've been in danger regardless of whether or not she came to earth (Marco excepted, since he's willingly accepted it)
I hate the episodes where Star runs amok causing death, destruction, or property damage to Earth over trivial nonsense whIle facing no consequences, like the bike and crushed police car episodes, or the one where her role model ran around breaking shit.
I liked everything involving Toffee, and I liked when her Dad came camping and wr got to see not every Mewnie is a complete psychopath. I like the exploration of the "monsters", and when it gets shown that she and her family are still at heart freaky fucking aliens with weird biology and values divorced entirely from Western democracy.