What is Mundo's Niche?
Tanky DPS.
Champions with a tank primary in roll are durable champions that must be dealt with because they bring a lot of CC/utility.
Champions like Mundo with a Fighter primary and Tank secondary are durable champions that must be dealt with because they bring a lot of damage.
His damage falls off to abysmal way before he becomes that unkillable regen tank in the late game.
Urgot's damage falls off.
Mundo has %HP damage (essentially doesn't fall off) a 100~200 AD steroid, and 2 sunfire capes perpetually going off. He can build full-tank and out-DPS a wide range of other champions late game.
He also cannot split push as well due to his weird mixed damage and poor wave clear.
Mundo is the single fastest speed-run champion I've seen. He's capable of pushing over the Nexus in less than 6 minutes in a 1v0 set-up, which is about 6 minutes faster than most champions. That's the exact opposite of poor wave clear.
Mundo was also picked as a jungler in season 2 specifically because his jungle clear time was among the fastest in the game.
You're doing something horrifically wrong if you think Mundo has wave-clear issues.
Mixed damage is also an advantage for Mundo. It's the type of damage you want on any champion building tanky - pure physical or magical damage is only useful if you're pursuing a glass-cannon build, which would be suicidal in Mundo's case.