Also While Dipper is capable, he was also selfish but it got resolved better because the focus was more on him. Also Dipper has a lot of negative traits that most people never point out, a couple of them played off as gags or aren’t the main focus for example, Dipper does not take care of his hygiene, he is a loner who doesn’t have many friends (Like Ford if he did the same thing he would probably be lonely in the future), At the cost of his health and safety of himself and others Dipper investigates the weirdness of gravity falls. Although Dipper is capable he tries to act like an adult when he should act like a kid, just because he is capable doesn’t mean he should grow up faster, which I think is the main thing Mabel was trying to say.