DON'T make Yuumi's Q damage AOE, but DO make the slow AOE
I feel like, even though it's a tiny amount of damage, making into an AOE damage spell would be to much for a champ that can sit in untargetability and harass from near 100% safety.
Now what I think SHOULD be AOE though, is the slow. Keep the single target damage but make it a tiny AOE slow.
Could still be oppressive with Yuumi's safety, but I think it's a good angle to approach from.