Poor 
.
We've got a tank meta, but every other tank does what he does: just better - be it post rework Sejuani or Mao with his "new" AoE ultimate.
Amumu does what they do: just worse.
His kit feels outdated and doesn't live up to the utility and flexibility of other tanks.
I know a lot of people hate the tank meta, yet I do think that poor mumu could need some love, a small rework.
he has no disengage or any significant defensive ability,just like 
his kit relies on items,tank items got nerfed and now theyre both unplayable.
Some random thought: He could get a new passive (like Kalista's) where he befriends other allies and bonds with them <3 (possibly a mini-knight's vow) Let's get poor mumu some friends.
Unpopular isn't synonymous with weak/bad. If you don't like his kit it doesn't mean he needs to be changed.
