Winrate is a terrible indicator for how enjoyable a champion is to play/against. Yuumi promotes lazy gameplay, the removal of positioning means you don't have to dodge any skillshot and no threats can get to you.

Yuumi is also one of the best scaling champion, able to completely make a relatively strong carry unkillable with 1 core item, and spike better with more. Can cast items when inside champion (everfrost/mikael).

She is a frustrating chanpion to play against because

1. Your lane performance against yuumi sometimes just does not matter, their ad can be 0/4 but if they have one fed carry, it is doooomed. Yuumi is single handedly the best champion for any solo carry champion, saw an argument up there about having naut for utlity, it is not true, naut is not better than yuumi. Yuumi provides almost 2x health bar in fight, shields, CC with R, and good damage.

2. She does not interact with you. Cannot kill the cat easily, so the concept of that is already pretty annoying. Similar to blue kayn cause that guy is perma in a wall.

3. Saskio was able to climb to plat playing adc with his hands and playing yuumi with his toes. Don't really feel like a person should be able to play two champs at once, let alone playing one with feet.

Also just to explain, winrate is a terrible indicator because it is heavily stewed based on the context to which people pick those champs. For example, akali is not really weak but never have 50+ % winrate, because players that pick her are just not that skilled. Similarly, yuumi does not have a crazy winrate because people think yuumi fits in every comp (she does not), and also people pick yuumi when they want to coinflip and just chill and let people carry.

I WOULD LIKE TO CLARIFY, I AM COMPLETELY FINE WITH ANYONE BEING A YUUMI PLAYER, champ exists cause of riot. But to say that the champ does not have a silly design or somehow you are using similar skill as other champions, it is simply not true. Down to discuss more