Well humor is a useful tool in putting people at ease. It relaxes students, and allows you to impart a deeper knowledge without being heavy handed.
On the other hand you can go too far in your attempts at humor. This could cause students to cease taking you seriously as a leader. I think that anyone who takes them selves too seriously as a person or martial artist is bound for disappointment. One will fail, and lose and do any innumerable things which can only be overcome with a smile and hard work.

hope that helps.