I believe in the value of subjectivity, and as such I always avoid trying to tell people how they should feel about something in this show (or anything, really), but this time I’ll have to make an exception:

Why is such a big part of the fandom hating on Marco?

Sophomore Slump started an arc of growth for Marco, who apparently was “left behind” by Star, as far as maturing go, in the last part of S2 and the very beginning of S3. Once home from Earth Star managed to put to good use all she learned from her year of growth, resolving to be the best princess she can be. On the other hand Marco was lost once separated from his best friend. This lead to him being kind of a jerk for two segments, while his ego got systematically deflated to ground level: left by Jackie, ridiculed on Mewni, scolded by Star for his feeling of entitlement.

THIS DOESN’T MAKE HIM A BAD CHARACTER, THIS DOESN’T MEAN THAT HE ACTED OUT OF CHARACTER. Star failed and suffered a lot during S2, and as a result grew up enough to defeat Toffee and improve as a ruler.

Marco didn’t grow as much, and now it’s his chance to do it: by falling to the bottom (literally, in the castle sub-basement), he managed to rise again, and I’m sure that this time he’s going to find a way to “make his own balloon”, so that with or without Star, on Mewni or on Earth, he’s going to be able to find his road on his own.

And he already started making up for it! Trial By Squire showed two of Marco’s best qualities, already starting the “redemption” part of his arc, after two segments of fall: his perseverance in pursuing a goal, in this case being a good squire to Star, because now he damn well knows that he should be grateful to her, that he’s lucky to be friend to Star,

AND his qualities as a friend. What does Marco do when Star tells him that, as a princess, she does not need knight equipment? HE MAKES SURE THAT STAR HAS FUN. Because this way he can be a good squire and a good friend at the same time, getting his knight, and his bestie, what they really wanted. SILLY FUN.

So if we could all forgive Tom’s lies and tricks because he showed that he truly wanted to change, and now he managed to put enough effort into it to actually get Star back, why in the world should we start damning Marco just because he acted like a jerk for two half segments in a row, when he’s already doing his best? Star wants to be the best princess she can be? Good, then Marco is going to be the best squire, and the best friend he can be.

You know, having characters actually succeed after having failed is the best way to show that they grew, and that they weren’t just words.