It’s a very colloquial expression. You can fill in the blank with certain general things that someone could do or be interested in. Examples: I’m not much of a sports person. I’m not much of a car person. In those uses the sentences don’t make sense without the person at the end. In other phrases with similar meanings person is not needed because the word filling in the blank makes sense without it. I’m not much of a drinker. A variant is I’m not a big car person. A similar phrase with a similar, but more general, meaning is I’m not a big fan of...

You can use it to say that you are not good at, or don’t enjoy, a particular thing. Example: “I’m not much of a camping person” or “I’m not much of an opera person.” Sometimes you can say it without “person,” if there is a noun that means “person who does X activity.” Example: “I’m not much of a camper” == “I’m not much of a camping person” . “I’m not much of a runner” == “I’m not much of a running person.” But you couldn’t do this with the opera example, because there is no noun that means “person who watches operas.”