I believe the argument isn't that ancient statues were ugly, but rather that reconstructions are ugly (unfortunately this has been used to argue against the now ascertained fact that ancient statues were indeed painted). Purely subjective judgement from someone not trained in the arts: that photo of the Augusto di Prima Porta doesn't look like a great paint-job. The idea that, like the statue itself, the painting must instead have been a great work of art lost to time seems solid to me.