He probably does a lot of damage to be honest. He can play the politics game and get protection from someone like Joffrey (who has a few screws loose himself) in exchange for wreaking havoc on his enemies. Meanwhile he spends a while with the keys to the kingdom and he concocts a plan to kill everyone. The dude is dangerous. And his scale isn't being considered in this rumble.
Joker goes against a human is far beyond every human in GoT on every level. There is no one on that show (book series) that has even a fraction of the intelligence and reasoning skills as Batman. And Joker can generally keep up with him.