He's the only Founder we even have a vague description of, but yeah, the monkey-like is funny. Perhaps JKR means to suggest he's got a shrewd look to him? Though what does it say about the man that he hid a giant statue of himself in the *basement*?

The only confirmation of the statue's identity is that Tom says so, right? He talks to Salazar while looking at the statue, but perhaps it would work just as well talking to the wall, or not speaking directly *to* Slytherin. It just seems more likely to me that the statue is someone else, and Tom just made that assumption. Or not. I just agree the statue is weird; if nothing else, you'd think it would be more flattering.