To be fair, the dominating conversation point when talking about Hannah isn't her appearance, it's her horrendous personality.

But I also don't agree with the idea that Hannah is meant to represent a normal or average girl with her appearance. She seems to deliberately choose outfits that are incredibly ill-fitting or unflattering. It takes effort to look that bad. I also think her sloppy and messy appearance is meant to illustrate her sloppy and messy life and personality. She is not someone who has her life together, and that's reflected in her inability to really dress herself.

It's not like she's alone in the crappy outfit choices. All the girls on the show have bad taste, but they have DIFFERENT bad tastes which reflect their different personalities.