Making a Holocaust comic book with Jews as mice and Germans as cats would probably strike most people as flippant, if not appalling. ''Maus: A Survivor's Tale'' is the opposite of flippant and appalling. To express yourself as an artist, you must find a form that leaves you in control but doesn't leave you by yourself. That's how ''Maus'' looks to me - a way Mr. Spiegelman found of making art.