When Nietzsche sez Plato was an ugly man, that is an adorable low blow. The idea being that just a very ugly man could have any interest into proving the material world to be an illusion, since then his ugliness would also be illusory. Turns out his name likely comes from a brutally big forehead, so, you see, the Moustache Guy was on to something (surprise, surprise). Even then explaining away someone’s world-views through such very paltry personal issues is a low blow.
Thinking about this, at first sight it looks like this is a vulnerability of serious thinkers, that by keeping high level arguments they unhappily open themselves up to coarser, blunter, stupid counter-arguments. That the personal stuff is just the basis for the arguments, and that those are, by their turn, more abstract and more important.
But there is a catch, there is cunning and deceit here. If the Moustache had said just that Plato is ugly and sad and dumb and he does not like him and, as we say in Portuguese, hates anyone who likes — well, that would have been like lowering our argument level. But what Nietzsche does is actually using the personal in order to raise the abstraction level. In pointing out ad hominem objections, he makes matters more complex and subtle. He uses a personal attack to make the argument more encompassing, not less.