dont judge yuumi so early and ask for buffs
yuumi is a very unique champion. Due to her uniqueness, it will take a while, maybe even a month or two, for players to understand what works and what doesn't on her, and her pattern.
I don't think they should buff her and then it turns out she is absolutely broken and we have to deal with a broken support for 1-3 patches.
Front page of reddit had within the 1st 4-6 hours "28% winrate yuumi" when it's just because she's a unique champion (most likely).
Give it time, and after players understand her, then consider buffs/nerfs.
Remember, even when you think you understand something, act like you don't, and delve deeper, trying to understand it.
The champion is a lot like IO from Dota 2. IO is not that popular/low winrate but is highly contested in pro play or banned (last time I checked). Until people realize that types of crazy strats/plays you can do, she will seem weak.