Joker didn't need to kill many people to be a strong villian, maybe it was surprising that he wasn't such a mass murderer as was expected.
Remember his scene with Harvey; "Think of what I did to this town with a few bullets and some gasoline . . ."
Well, compared to other villains, the Joker turned out to be a sadistic SOB. I mean Iron Monger...did he even kill anyone? I know Jack's Joker tried to kill a bunch of people at the end, but Batman took the ballooooons away. "My balloons!"
Anyway...it's hard to be a villain in a PG-13 movie. I mean, the Joker out-killed the guy from SE7EN.