[...Is that...singing? Why is that green guy singing?

Kovu doesn't actually say anything, he just eyes the human with some amount of scorn.

Lopmon, though, seems to be getting into it, and is wiggling around a little with the beat. That gets Kovu to look at his partner instead, the look of scorn outright becoming one of disdain. Lopmon stops dancing, and gives Kovu a sheepish look.]