Why doesn't Chochem volunteer to be Santa Claus, besides the fact that otherwise there would be no movie? He's already a wise immortal being, possibly with supernatural abilities, who cares about the well-being of Martian children. In any case, he probably would do a million times better job being in charge of delivering presents to every child on the planet than the "laziest man on Mars."
He didn't strike me as particularly jolly. A Santa like that would probably make the martian kids even more depressed.
He also seems like he can barely get through a conversation without keeling over. I can't see him shimmying through vents, or building toys even if the production is automated.