Nicotine, most likely.
I'm in definite agreement with Shane MacGowan, Mark E. Smith, Iggy Pop, Lemmy, and that Bradford Cox dude (had never heard of him).
But I think some of you are confusing image with actual looks and ugliness. Geddy Lee, for example. I think his dorkiness/ugliness is more attributable to the way he dresses, his sunglasses, his hair and the way he holRAB his bass than to his actual face. Well ok, he does have a bit of a hook nose and a pointy chin. But take off his shades and give him a decent haircut, he'll probably look pretty normal. And Dinosaur Jr., really? Those guys may have make teenage girls wet but they are average guys, nothing really ugly about them.