Because of the recent nerf on patch 10.12, Yuumi now has one of the lowest win rates on patch 10.13.
Up until patch 10.13, Yuumi was one of the most hated champions in League of Legends. On previous patch 10.12, the Magical Cat had around 78% ban rate worldwide in ranked games.
Although her ban rate in Korea was nothing significant, on EUW she was a 100% pick or ban champion and on other servers, her ban was above 60% as well. League communities were full of complaints and players were demanding for her nerf.
This eventually happened on patch 10.13 where Riot decided to give her some significant nerfs. They reduced her E mana cost but added a new effect which now cost her an additional 15% maximum mana every time she cast her E. The goal of this nerf was to prevent her from spamming heal nonstop and eventually it seems like they are kinda successful with this changes.
With the help of this nerf, she went from being the highest ban rate champion to one of the lowest win rate champions on patch 10.13.
Currently, on Plat+ Elo, Yuumi only has a 45.13% win rate with a 9.9% pick rate, however, her ban rate is still around 50%. In Korea, she only has a 43% win rate making her the lowest win rate champion in the Korean server. But surprisingly, her ban rate in EUW, EUNE, and TR is still around 68% according to u.gg.
She might be one of the lowest win rate champions, but there are many players who still think she is overpowered where others think Riot kinda over nerfed her.
But if her win rate continues to drop like this, even with a high ban rate on some specific servers, Riot Games might give her some changes in the upcoming patches.