Jesus give her a little break, she is just a kid. What do you want from her, commit seppuku because Bill took an advantage of her when she was the most vulnerable? No one is perfect. Dipper has the same sins, yet no one will say it's his "established character". The show didn't need to show her remorseful, shit was done, now she had to fix it by beating Bill.
Even before that, she mostly got away because her brother allowed it (like when they were saving Mermando or when they were travelling in time). Yes, she is spoiled, but by no means "unredeemable". It's fine to make mistakes, that's what makes us human. And humans are suspectable for change.
You can consider her bad traits in her overall character, but it doesn't mean she must be always like this.