That seems... a little out of character to me, actually. It's certainly sick enough for a Joker crime, but from what I've read of him, he loves it when people actually laugh at his jokes. The thing is, people so seldom DO laugh that he usually winds up going 'aww - you're no fun!', and trying to spruce up the situation as only he can. But people ARE laughing here - in point of fact, they're cheering and applauding! I think it would be more in character for Joker to kill the clown, take his place, and then take note of the ones who DIDN'T laugh and ambush them afterwards - or be pleased at the ones that laughed the hardest and 'reward' them in ways that he thinks of as a reward, but they wouldn't - like, if a little boy was asking his daddy if they could take the dancing elephant home, he'd give him the elephant - dead, in the middle of the living room, with a Joker grin carved into its side. Punishing the ones who DO laugh seems a bit topsy-turvy, even for him.