I think as long as you have the time to put into it, it doesnt matter so much how active you are as much as how supportive you can be for a new student. Of course it doesnt hurt to know how to get your mentee the hook up for tests and parties and whatnot, but its not required.
I am very involved on campus but I am not going to mentor, mostly because I dont feel like I can make the time commitment and I dont want another elective. I might become an un-official mentor if it happens that way though.