Just in case it wasn't clear , you're not supposed to play that cat (yummi) with an adc
It's a support sure, but i'm pretty sure a support that is not meant to take hits is not supposed to peel hyper dependent adc.
it's now min-1 before you realise yummi will be played with
because ap boost is way better on them,
so no stop calling her underpowered and play her the way she is supposed to be played.
edit: Read her kit before talking about things you don't know
when attached to a champion she Either
boost her own AP and her ally's AP by 5 / 9 / 13 / 17 / 21% IF ally is mostly AP OR boost her own AD and her ally's AD by 3 / 5.4 / 7.8 / 10.2 / 12.6% IF ally is mostly AD
so who ever said that it was better on adc , do you realise she gain a percentage of her ally's ad but cannot use it ? Whereas if she helps aan ap she can use her ally's ap scaling as a buff to herself.
She is a living 
that are way easier to build than
+
/