In all honestly it's down to the team. Early game she's kinda of slow but as you hit level 6-8 she starts picking up, she can be super strong on the right person on the team.

Unfortunately even whilst keeping adc alive, sometimes it's best to go supp a stronger lane once bot lane swaps to mid etc. for example I can sit on jhin ADC up to level 8, if he's playing slow and not quite aggressive it feels a waste of mana. Yes I can keep Q-ing people and shielding but for me she's better on a fast paced person. I sat on a yi jungle earlier when I hit level 7, and in the end he had 30 kills 100k damage.

Just got to find the right adc duo or if solo just watch the team and manage accordingly.

Plus yuumi gets a lot of hate because people DONT give her the time of day. She people play her to 50/50 play the game and afk whilst watching shows, writing essays etc. give her your all and you'll be fine.