Probably Kayle and a few other champions who just...beat him.
Looking at LeagueofGraphs, we see Mordekaiser has 5.5% higher Winrate against Urgot. Yorick, Kled, Heimer, Wukong, Jax, Sion, Viktor. All experience at least a 1.5% higher winrate than their norm against him. Sion has a 4% higher, Wukong and Viktor have a 3% higher. Kled has 2.5% higher.
However, few of his counters are actually in the meta. Sion and Viktor are all that appear in proplay, and Jax is added to that in soloQ.
Everyone that beats him well does so rather logically. Urgot is going to have a hell of a time killing Morde through his healing and shield, while Morde's ult rips through his health (because Urgot normally stacks a lot of health, but not a lot of resistances), his W heals minions to make it take even longer for Urgot to attack him if he doesn't land Q, and his overall DPS is just going to be a hell of a lot higher. Jax beats Urgot because he presses E and laughs at Urgot's damage. Kled beats Urgot due to repositioning with E and %hp damage with W, while being a colossal pain in the ass to kill even with Urgot's ult, because Kled cannot be killed by a single attack if he's on Skaarl, meaning even if Kled's at 2% health, as long as he's on Skaarl the execution will just dismount him. Yorick again has %hp damage (although not as much as Kled and Morde) and has minions that make Urgot's W almost impossible to hit with if he doesn't have Q. Heimer does normal Heimer top things, but, again, summons that make landing Q absolutely vital to actually hit with W, and he gets Zhonya's as a core item anyways.