There are some issues with this build.
#1, your masteries are bunk. I recommend you take the 3 out of mercenary, and 2 out of magic resist, and put 2 in indomitable. You should also take all 6 points out of offense, and put them in utility, going for 2% MS and enhanced buff duration. Why do you take summoners wrath when you go flash smite? You should put 4 in movespeed and 1 in buff duration. 2% ASP and 3AD is not worth skipping utility.

#2 I think you need to rethink your marks. I think magic pen is the way to go. Attack speed will SLIGHTLY speed up your jungle, but not by enough to be worth it. I go attack speed marks on junglers like nocturne, shyvana, cho'gath, and malphite, because they get BIG BONUSES from their attack speed.

#3 your skill order is wrong. I appreciate you want to gank at level 2, but 99% of the time, that's not an option. You can instead do a fast smite-less blue after wolves, do wraiths, and go to red, and be ready to gank at level 3 with your bandage toss and double buff. If you skip your jungle to gank and are unsuccessful, a smart enemy jungler will see you don't have red and he will go steal it. Is screwing your whole jungle worth forcing a flash early? Also, you're supposed to max tantrum, 100%. It keeps your health up in the jungle and it does WAY MORE DPS overall in the jungle AND in teamfights.

#4 Your items are fine, but those aren't what I would build. I prefer a hybrid dps/tank mummy, with double gp10->sunfire->abyssal, but I suppose to comes down to preference.

I don't play Jungle-mumu usually, and you really opened my eyes. However, I would like to point out that Tantrum Should be maxed first ALWAYS. While it isn't a disable, it's amazing in team fights as it deals AOE damage, it gives you massive amounts of aggro, it's very spamable, and it reduces all physical damage. I would also like to say that I find that slow-mumu is the way to go end game (building movement quints is good for this). That's Rylai's Crystal Scepter, Frozen Mallet, Mercury's Treads, Shurelya's Battlesong, Randuin's Omen, and Force of Nature. It's fast and secures kills like nothing else. Overall, great guide. +! :)

Well, you have a good guide but still, you have some minor problems:
1. You don't have best runes, instead of investing in AS, put some points in Bladed Armor , it is good for every jungler. You should say if there is any difference if you are lvl 20- on playing Amumu.
2. You max your Tantrum at lvl 14. Is that a mistake or do you have a hidden secret?
If that is a hidden secret, I can assure you haven't played so much League Of Legends.
3. Amumu is kind of a tank. You won't get kills and in order to keep up with the gold income you need to get phil stone and Heart of Gold. Later on turn the Heart of Gold into a Randuin's Omen.
4. Some time ago, there was a Skarner guide, it was named "Support Tank Skarner".
He had so original things in it about jungle, something like:"Jungle Match-ups","Ganking Mid Lane(telling how to gank every mid champion)","Ganking Top Lane", "Ganking Bot Lane", "Counter-Jungling", and all kind of stuff, I think you should make something like this too.
By the way, I like how you keep the phil stone until very late in the game to have a bigger gold income :3.
You deserve a +1 but please look at my review and change some things, if you think you needen't, PM me and explain your reasons.