Perhaps Rumpel just liked the idea of an earl’s daughter (I don’t think Maurice was a king) having to do servant work. When he was sat at the table telling her what her duties were, she was very accepting of it, something you wouldn’t expect someone of high birth like Belle to be like. He threw in the quip just to probably throw her a curve ball, see how she’d react and when he told her it was a joke she seemed to relax a little. Belle also treats Rumpel like another human being, nobody else seems to.