I have been playing mundo here and there for the past year or so. Currently he is really strong. A few patches ago his Q got buffed to refund his health on kill. More recently, before s6, his E got buffed to scale with his max health and reset his auto. These buffs did not by any means make him op. What has made him OP is the new masteries.
I go 18 into resolve taking all the health regen i can and "Grasp of the undying," which scales of max HP. I then go 12 into ferocity grabbing spell vamp and feast. With the new masteries killing a target with q heals you. In previous seasons you could achieve this, but you need spirit visage and runes. If you are low cleaver farm under tower and heal up, along with your passive.
While he is easy to counter, ignite, morellos, and the new ad grievous wound item, he is strong right now. Although i hate to say it, he probably needs a nerf. While he can crush early-mid game he falls off late, and his win rate plummets after 30 or so minutes.