Mundo is in god tier because I need 1 tank in god tier. Blitz is a half-tank, but he isn't really a full tank. Mundo has a ranged harass that hits really hard, he has innate steroids that means he only needs defensive items, and once he hits 6 he can basically sustain forever. He is a punching bag, but you cannot leave him alone otherwise he will solo whoever he targets on your team. He is the best kind of tank. The one you can't kill without concentrated effort and the one you can't ignore. Oh, and if you are feeling a bit of heat through the enemies Ignites/Morellonomicons, maybe try going Cleanse as a summoner spell. It won't help all engagements due to its CD, but it certainly swings the important ones where you all in them.
He has no hard CC to offer in fights, and no ability to peel for the carries. When late game hits, he is just a big chunk of health sitting in the middle of the fight. I really don't see how he can supposedly solo a champion, even with his AD steroid.
There are other options out there if you want a God Tier tank.