Dance if you're happy
After a 2nd viewing...
Arthur/Joker's dancing was totally logical, from his point of view.
Dance without concern when your happy.
He'd killed 6-7 people when he danced down the stairs.
He was happy about it, he didn't care, he was free and cheerful for the first time in his life.
He did a little dance when he saw the cops getting hurt.
He did a little dance after he killed DeNiro.
And after he killed the shrink at the end.
He's batcrap happy and free as a bird, in his mind... why shouldn't he dance?