A mole can mean either (1) a small animal that lives underground, or a second meaning which is (2) a spy who works inside of a company or group to steal information. From context, I believe it is saying "Batman is not a spy", using the second meaning of "mole". I would be positive that was the answer for anyone else, but for Batman there is an enemy called "Mole Man", so it might be a reference to that person. You will need to look at the context in the comic book/TV show/movie you are looking at.

Calling someone a mole is calling them a spy. Proper grammar would require that "no" to be a "not," but this reflects the way some people speak.