What? Mundo is lit. Lol. He doesn't bring anything new to the table, ofc not he's fucking old, but his playstyles fairly unique. He's the only "regen-tank". His kit doesn't not have synergy. He needs to be in melee range/deep in the fight for his e and his w. His ult allows him to stay deep, and gives him the movement speed to get in there. His q also helps him get into melee range. It kind of fits. All of his skills drain his life. He has multiple ways to get his life back. It's not an amazing design, but it's not garbage. Maybe I'm biased because I like the champ. He's, in a sense, the rammus of APs. He also does shit loads of both magic and physical damage. If I play him I normally end the game with moat damage taken and most damage dealt.
His balls deep "I can't die because I'm healing, playstyle is unique. No sound in game is as satisfying as the thwack from his cleavers. Lol.
I play him because he's an "AP rammus", a tank, a physical dps, a magic dps, and is capable of carrying.
I already talked about the synergy in my first rant. I likely haven't convinced you of anything, but have you played him? Most tanks "can't die", deal meh damage, and have good cc. Mundo "can't die", deals great damage, and has low cc. Imo he's the only champ of his kind. Most comparable is shyvana, and that's still not very close.
Edit because people apparently are mad if mundo gets called a tank. He's not categorized as one. He doesn't have a huge defense steroid like real tanks do. He is one of the best damage soaks in the game, though. He's a fighter that tanks better than most tanks do. So call him what you will. Lol.