Mundo is kinda just OP right now. I think people have this misplaced powerspike on champions that are known to have had a strong lategame before a major change to them. Remember Ryze? People were calling him a "lategame" champion when he was dominating the early game with his perma snare and would actually fall off later due to people being able to effectively lock him down and focus him. He was nerfed a few times and now he has a pretty weak early game (in general) and a pretty strong mid to lategame, after about 25-30 minutes. The same thing I think is happening with Mundo to some extent, but I think there is something else to this statement.
Look at other junglers right now. They are primarily early game pressure junglers who try to snowball the other lanes with their presence. Mundo on the other hand, is a terrible ganker and doesn't really do anything except counter-gank, invade, and farm his way to the midgame where he will be a very dangerous force to be reckoned with. So really, he DOES outscale every other jungler because he is meant to scale and be a real threat himself as opposed to his teammates when it comes to mid and lategame. Given I think that all Mundo players take Strength of Ages and Cinderhulk in the jungle, he will have a very impressive midgame which is where a lot of junglers start to fall off.
Bottomline. I think Mundo is OP right now, particularly in Solo Queue because he can carry so hard based almost purely around his kit's power. The E change was so substantial to him and I don't think many people realized how much that change would affect him. We now have a Mundo that has CONSIDERABLY more damage (particularly burst) and due to preseason changes, has become very hard to kill.