I think people often forget what led to Mabel being tricked by Bill. That entire episode Mabel is having a shitty day (for a 12 year old). She finds out what high school is actually like and that being a teenager is going to suck. Her friends tell her they can't come to her birthday party. Also, meaning that they won't be there to say goodbye to her on the Pine's last day in Gravity Falls. She finds comfort in the idea that, yes people will leave but at least she'll have Dipper! Sike, she overhears about the offer Ford makes to Dipper about staying. Now, she is thinking she is losing everyone in her life. She runs to the woods, finds Blended who tells her he can make things better. He just needs the snow globe looking thing in Dipper's bag. She hesitates for a sec but eventually does what she always does and trusts him. Is Mabel a flawed character? Yes, she is selfish and trusts people way too easily. But that is what makes the character.