She's not a sex symbol, that much should be obvious. Why is this even a question? Men define what is attractive to men, not women. And since beauty is universal, everyone knows she's a fug.
She just has a lot of fug sympathizers, along with more attractive girls watching the show because it makes them feel even better by comparison.
Lena Dunham will fade, just give it some time. Try as she (and her fat sisters) might, there's no escaping biology.
I downloaded the first season and spent half a day watching it on a Sunday for my research purposes (and am in the process of writing a post on it), and I wonder if anyone else noticed something funny but true: that fugs slut themselves out WAY more than attractive girls because that's the only way they'll get attention. In the series, Lena gets naked way more than any other chick (and it really does make you vomit a little), while arguably the most attractive (yet portrayed as ditzy/awkward) girl in the show is a VIRGIN (yea right haha). There's 1 episode where she strips down to her undies and holy smokes she's bangin. No further explanation required.