the fact that she is super poplar, everybody players her, and still maintains the HIGHEST winrate says a lot. compare that to malzahar, heimerdinger, skarner - few play them, but excel when they do.when a champion has one of the highest ban rates, the highest pickrates of junglers champions, and one of the highest winrates, i wouldnt say "overrated".
you cant compare winrates and banrates. the highest ban in the past month is katarina, who has the 10th highest pick but a 45% winrate @ 47th place. why do people ban her? because it only takes 1 under-preforming midlaner to completely wreak your team. compare that to sejuani, who has almost as high a banrate, a higher pickrate, and one of the highest winrates.
compared to amumu, she provides more AOE CC (AOE-capable knockup and AOE-capable slow versus just a single-target stun), does only slighty less damage, has slighty weaker initiation (duration-wise), but stronger initiation (enemies cant cast spells while locked). She doesnt have quite the range, but makes up for it by using her Q to escape and E to kite. she also isnt as mana hungry. her ganks also dont rely on a single-target skillshot, so its easy to hit with a flash. pre-6 if that skillshot does hit, she can follow it up with a slow. amumu cant.