Susan was expecting more of a fight from Lauren, and so he's actually caught off-guard by Lauren... Basically giving him permission to sing it? That throws him off more than anything else, since he was really just being silly and teasing Lauren about it, but he doesn't really mind. Of course he only barely remembers the words Joker was singing anyway, but he has a vague enough idea that he can at least make an attempt.

And so after a minute, and with another glance to Lauren, he starts to sing. He has the tune right, and the vague idea of each verse, but... Yeah, the words are wrong. At least he knows it's supposed to be crude though so his substitutions and places he wings it are still fitting.

Have fun listening to that, Lauren.