Girls

[The Summoner of Leviathan]

UK Skins > Girls.

I think as a body type she reflects the average American woman much more than Marnie or Shoshanah. Quick research shows that the average woman is a size in the USA is 12-14 and that this size range is less likely to wear clothing appropriate for their body shape, especially those who were previously of a single digit size and are now double digit.

I don't think her behaviour reflects the average American woman. Hannah has obvious OCD problems and neurotic tendencies.

References:
About.com on woman sizes
Woman Sizes and Appropriate attire
More stuff on average woman size in the USA

I don't think Hannah is necessarily supposed to be "beautiful", she is just who she is. I think she is comfortable in her own skin and if you think that is beautiful then that is up to you. But I don't think it is the case that the audience is supposed to think that Hannah is beautiful or believe so.