Hey bud. Gold 5 Amumu main here, THOUSANDS of games on Amumu. Love my mummy.
You're absolutely right. He feels terrible right now, and it makes me sadder than the sad mummy himself.
The first thing that really killed him was the nerfs to his W's percentages. Made his clear slower, made his damage less meaningful, made him basically a one-trick ult machine - but his ult was still really great. (still is, kinda)
They also nerfed his AP ratios before that (or after? I can't remember) which made him even worse.
They also changed his ult from being a stun to snare.
They also nerfed a few of his key items, which indirectly nerfed him.
Abyssal mask was good for him, but too little too late.
If I'm not mistaken, at some point they upped the cooldown on his Q, too - but I can't verify that. I think they did, though.
Basically its been nerf after nerf after nerf after nerf, and the sad mummy is in a pretty...well... sad state right now. I think they could probably buff his %health damage on his W back to where it was, or maybe increase his AP ratios (although you shouldn't really be building too much AP on him anyway.)
That or make him tankier. Right now, unless you build absolutely full-tank on him, he's pretty squishy. Building AP on him just makes him a glass cannon CC Nuke. You go in, you blow your load, you die. Building hybrid on him just feels bad, because you're not really doing meaningful damage, and you're also not tanky enough to engage (his main job) so hybrid builds are horrible on him.
So as it stands he's a completely ignorable tank with a decent ultimate at best, or a kamikaze ult machine at worst. Neither of which feel particularly good to play.
Make his E do additional damage to jungle creeps or something, i dont know. I'm not a game designer, and I'm not good with numbers. Something's gotta give, though. I'd love to be able to play my favorite champion again and feel good playing him, and not have my team pursing their lips at me when I lock him in because there are CLEARLY better choices available.
Sad mummy is so, so very sad.