I know that right now Yuumi seems a bit weak when you look at winrates. But what did you expect from a champion that was JUST RELEASED?
Give people time to find the right builds, runes and playstyles for her before judging her as underpowered or weak. She is not the kind of champion you can master in one day. Her kit has a lot of small things to take into concideration when playing her, like how she should proc her passive safely.
To be honest, it scares me that people demand buffs or changes on day one. Do you want an overloaded champion, because that's how you get one.
Ps. I've had a blast playing her. She is hard and I haven't won a single game yet (Played 3-4 games with her by far) but man is she fun to play. I've been testing runes a lot and I can't wait to experiment more.