Question: is amumu a good jungler?
Answer: IMO he is not the worst jungler by a long shot, he will never be terrible due to how strong his R is (there are plenty more atrocious choices, believe me I've tried). In fact, right now Amumu should be decent, since tank junglers (Zac, Cho'gath, Gragas, Sejuani) are coming back in vogue due to Cinderhulk buffs.