do you really think is it good designed heal, i mean the worse your adc trade you are more rewarded ? Played few times against her if you poke her and they are hugging turret you will lose in sustain battle, she heals more than soraka almost. Anyway well designed champion, i do like her kit and unique playstyle. Only problem have with her E(heal)
The heal scaling dependent on low health is specifically to encourage the Yuumi player to stick it out and fight with their ally rather than leaving as soon as they're low. Your best chance to turn fights is in the most dire of moments!