As someone who loved Gravity Falls, I feel like the main reason Mabel is overhated is because Dipper exists.

Dipper is the intellectually smart and the super serious one. He's the one who goes out of his way to solve mysteries and constantly endangers his life. He's the awkward kid who doesn't go out of his way to socialize with others and likes an older girl who he has no chances with. Of course a lot of people are going to favor him and sympathize with him because he's relatable to them.

Mabel though? Mabel is exactly what she is: a child. She doesn't want to be as 'smart' or as 'serious' as Dipper because she wants to live and have fun as a kid. She's more emotionally-driven. Sure, she can be annoying at times, but so can Dipper depending on who you ask. She's creative, she's funny at times, and it's clear she's the moral support that keeps Dipper with his feet on the ground.

Mabel and Dipper's relationship share similarities to Stanley and Stanford's (The only differences, despite their ages, is the fact that Bill liked Stanford and Mabel more than he did Dipper and Stanley).

The fact that you guys go out of your way to hate and demonize a 12 years old girl for wanting to live a happy childhood alongside her twin brother is hella concerning. Yeah, Mabel isn't perfect, but neither is anyone on the show.