The phrase "not much of" in the negative, is how this is used. So, "He is not much of a doctor". It means he is not a very good doctor. I don't believe it is ever used in the positive sense. "He is much of a doctor" `is incorrect. "Much of" is used all the time, but this is different. It refers to a large part of something such as "Much of the time, it rains in England"