Right now, it's well-understood that Mundo is underpowered on Summoner's Rift. This is in large part due to his heavy dependence on reheals (his passive and his ult). Not only do they expose him to Ignites, which are very common, and the many abilities and items that block heals, they push him toward a tanky build that doesn't work with Mundo's best abilities. Without a harder CC, Mundo cannot do anything as a tank, but his design makes that the most obvious solution, which is a problem.
Of course, this speaks to a larger issue with his design that would require a more complete rework. Instead, I want to address Burning Agony, which is a quintessential one-point wonder at this point. Not only are Cleaver and Masochism just much better, Burning Agony is too expensive to be useful. And without hard CC, even tank Mundos running W constantly can't do much in team fights. Burning Agony's primary utility right now is clearing creep waves, which is okay but unspectacular.
This pales in comparison to the problem with his CC reduction, which makes no sense. If the best Mundo builds can't run Burning Agony constantly, they need to be able to get the CC reduction at the times it matters most - for escaping the strongest CCs like Stun, Knockup, and Fear that both block Mundo's recovery ability (Sadism) and his ability to escape death. But you can't activate Burning Agony during these effects, either. This means that Burning Agony is only useful in situations where its effect is already marginal.
So my recommendation is very simple: make Burning Agony's CC reduction a passive effect (with a small decrease in effect to compensate). Not only would this give incentive for DPS Mundos to put more points in it, it would counter this clear design flaw.