Fuck you OP, I can't believe I'm writing a dissertation defending this shitty cartoon.
Mabel is an almost 13 year-old girl who just had an extremely terrible day, only for her worst fear to come true.
I've worked with kids at this age, they'll have a mental breakdown over anything because they're terrible at processing their emotions. For children, there are three general responses to trauma, which are fight, flight, or freeze. Kids can't rationalize their way out of a paper bag, they either cry, scream, and yell, run away and avoid the situation, or they shut down completely, say and do nothing, and let it happen.
In other words, being in a very agitated state, hearing something bad, freaking out about it, then running away to hide to process it is a valid response from an almost 13 year-old, especially without an adult around to step in and act as a moderator.
I often get the impression that Dipper is written with the intent of being more mature for his age, but ironically, that only strengthens how young and naive he is. Dipper has a strong need for validation from those who are older. He wants an older girl and he wants Ford to like him. He's socially awkward and doesn't make friends with kids his age, so he compensates by acting older and being around those who are older.
In my work with kids these age, these are the ones you need to watch out for. They will cling to you and refuse to socialize with other kids their age, and if you're not careful, they can get too close for legal comfort. As an adult, you have to disengage as much as possible with them, give them as little special treatment as you can, and look for signs of abuse, as a child imprinting on adults that hard, in the real world, means something, somewhere, is abnormal.
tl;dr Mabel acts her age, Dipper acts like a future child victim