What makes an mg protagonist interesting?

They don't even have to be interesting or exciting, in the sense of 'quirky' or 'unusual'. They can be the personification of 'everykid', as long as they're involved in interesting things. It's how they relate to the story that appeals to me.

I can be attracted to any MC depending on the writing, honestly. I've enjoyed books with spunky MCs, with out-for-vengeance and humorless MCs, with MCs who are essentially the "everykid", with precocious MCs who are smarter than the adults around them...

What I would really love to see more of, however, is a vulnerable, earnest MC a la WONDER, or an unabashedly whimsical and girly MC like in FURTHERMORE. Haven't been able to find many MCs like them, though.

I think one that shows growth and is relatable.

It's not really any different than YA or A. There are of course things that appeal to children more so than adults or teens, but there are plenty of intelligent and complicated MG MC's out there.

Depends on the type of story but in general you want the protagonist to be relatable or interesting and show some growth/change.

The main character is different than the supporting cast. The main character is wrapped up in the story. They're dealing with the story problem. Whereas every other character in the book has a mystery to them because they don't necessarily overcome a flaw and/or a problem because that's the main character's job. Sometimes they do, but they're smaller than the main character's role.

If there's more than one main character, the same thing comes up. The main characters solve their foibles, the supporting cast not yet.

When the main character is a three dimensional person. Not a "kid". Not someone that represents a theme or value. A complicated being. Fully rounded with flaws, good traits, complex thoughts and a sense of humour.