Mabel is the only person in the cast who knows who Dipper was before he found the journal.
And that includes the audience. 5 minutes after we first meet the kid and he’s already found Journal 3 in the woods. We don’t know what his home life or school life is like. We don’t know what his relationship with his parents is like. Neither does Stan, Ford, Soos, Wendy or anyone else in Gravity Falls.
But Mabel does.
She’s the one to assure him he’s a hero with or without the journal. Because she’s known him her whole life, she knows what he’s capable of, and she knows he doesn’t need the journal’s validation because she sees it in him every day.
Now, not only did Dipper obsessively want the journal back, but we have the Author himself thrown into the mix. Dipper is clinging to that validation more desperately than ever. And that’s really starting to scare Mabel. Dipper’s become lost in himself and she can’t pull him back down to earth. She can’t make him remember all of his good qualities he prided himself on prior to investigating Gravity Falls: His bravery, his loyalty, his selflessness, everything he doesn’t need that journal for because that’s just the twin brother she knows and loves. If anything he’s become way more selfish since finding the journal and realizing “Ha! Look at me go, I was right!” He’s put the whole town–his family–in harm’s way many times just to prove a point. He’s become someone Mabel no longer recognizes.
More to the point, think about this next time you accuse Mabel of antagonizing Dipper. She knows him better than any of us. Yes, sometimes she takes her teasing too far, but other times she teases him because she knows he’s taking himself too seriously. She’s knows he’s being ridiculous at times and is trying to get him to laugh at himself. Because she knows him better than anyone. Even better than he knows himself.
Everyone, including us as viewers, only know Dipper over the course of this one summer. Mabel’s known him for almost 13 years. They grew up together. She’s privy to more things about Dipper, even certain things they’ve agreed to not talk about. Because that’s what being close siblings is about. You keep things between each other that not even your parents are aware of. You gain each other’s trust that way. You gain a better understanding of exactly who someone is in that way.
Mabel ain’t no angel. But she’s the one who’s going to save Dipper from himself. Because she’s the only one who can.
Worth noting that Alex did at one point confirm that Mabel teases Dipper when he’s being too serious, his head has gotten a little too big, or he’s not acting his age (meaning acting older than he is)