I have yet to play mundo into what I would consider a tough match up. Its just a very slow paced start that most people aren't patient enough to deal with.
The reason he works is that he just gets so much sustain that you can't force him out of lane and even if you push the waves under his turret, he is still able to CS VERY efficiently.
Here's his sustain:
- high base health regen plus recovery mastery plus perseverence mastery
2)passive which gives 0.3% max health regen per second
- Last hitting with cleavers heals you (with runic armor mastery)
- take health/5 quints
5)Ulti (post-6)
- all of the above increased by runic armor and spirit visage
Notice how much of that you actually get to start the game with. If you take a little too much damage on a trade, just step back, and cleaver farm yourself to full health while you regenerate 3x more health for free than the enemy laner. The only way you die early is if you get greedy or just get out of position.
Meanwhile, once he gets a full armor item (or spirit visage vs AP) he immediately has dive/turn potential. His E gives him significant flat bonus AD plus scaling Bonus AD based off percent missing health for 5 seconds on a 6 second CD without cdr (oh and its an aa reset as well btw). He also has CC reduction on his W (which is actually very over looked).
Basically, I let them push early, but i end up with the same CS. They over extend and get ganked. I have the CS advantage, but I continue to let them push. Somewhere soon after, I hit 6. I use Ghost first to initiate and trade. Then I either ult and dive or ult and zone. At this point I've either gotten a kill and a large cs/xp advantage or I'm zoning them off the frozen wave and getting a large cs/xp advantage. At this point you own the lane. Once you have an appropriate mix of defenses and health, it will take 2-3 people to stop you from pushing and diving.
EDIT: I think I only have 9 games with him in ranked but I mean I'm 8-1 over those 9 games and have had similar experiences when I was testing him in norms against all the toxic top laners.