Oh look at me, I'm Yuumi. I'm just gonna throw out a whimsical looking q, heal a couple of times and W onto my adc and do nothing for a 5-20 seconds. I've seen more interaction from shaco support than this champ. I'd rather play with Meowkai support than this. Even if Riot buffs her numbers, I still wouldnt want to play her. She's just missing something. When you play as Yuumi, you are just an item for your adc. If your Vayne is 7/0 in 10 minutes, all you gotta is W onto her and leave the room to do something like using the bathroom, getting a drink, or some shit and come back.
If anyone finds her fun, hey dude more power to you. But to me, I think Riot should seriously look into her kit once more.