I totally get where you guys are coming from, but it's weird to me that you're judging someone because they don't "measure up" to society's (or your) standards of appearance.
She obviously just doesn't give a fuck. She "goes caveman." She doesn't need to look good for anyone. That shows a large degree of confidence or inner peace, and that sort of aura or persona or whatever you call it is attractive. I mean, you can tell what she's like from the pictures - it's not just her appearance, it's her charisma.
Or maybe it's because Scott is narcissistic that he's into her; opposites attract, you know?
EDIT: In terms of not getting dolled up for a high class party, who cares? If she's happy with how she looks, why should she change it for some Hollywood affair?