I agree with Captaindemotion here. The Joker has been portrayed in many different ways over the decades. In 1940 or so, he was a cold, ruthless killer and mastermind whose clownish appearance was an ironic contrast with his somber villainy. By 1944, he'd transitioned fully to a flamboyant thief, kidnapper, extortionist, and criminal performance artist who was as jolly as he looked and who rarely tried to kill anyone other than Batman and Robin -- basically the same version Cesar Romero played in 1966-9. In the '70s, he became a mix of the early-'40s killer and the post-'44 laughing prankster, and the idea that he was "insane" was added for the first time*. (There was actually a 1953 story where the Joker pretended to be insane until Batman proved it was just a ploy.) This version as defined by Denny O'Neil and Steve Englehart was the foundation for the Mark Hamill Joker on Batman: The Animated Series two decades later. And then Frank Miller's The Dark Knight Returns and Alan Moore's The Killing Joke took the Joker to extremes of mass homicide and horrifying sadism that have basically defined the character ever since. And there have been other variations in recent years, though I'm not as familiar with those.
So it can't really be said that a novel portrayal of the Joker is "wrong." He's been many different, often contradictory things, and new variations keep getting added.
*(I put "insane" in quotes because the Joker probably wouldn't meet the legal definition of insanity. He understands the criminal and immoral nature of his actions and the fact that they harm people; he just doesn't care. So he's psychopathic rather than psychotic. He really should be competent to stand trial -- but then he couldn't keep escaping from Arkham to menace Batman again, so they fudge it.)
So it can't really be said that a novel portrayal of the Joker is "wrong." He's been many different, often contradictory things, and new variations keep getting added.
*(I put "insane" in quotes because the Joker probably wouldn't meet the legal definition of insanity. He understands the criminal and immoral nature of his actions and the fact that they harm people; he just doesn't care. So he's psychopathic rather than psychotic. He really should be competent to stand trial -- but then he couldn't keep escaping from Arkham to menace Batman again, so they fudge it.)