So Yuumi's AA range is 500, while most low range ADC have 525 and most range support (Brand, Lulu, Lux, Neeko, Nami, Sona, Soraka) have 550.
There's a few reasons I have issues with this.
1: Her passive is completely useless if you can't Auto, it encourages you to try to poke with autos. But you are at a massive range disadvantage in lane to get even a single auto off. It basically means that you'll get auto-traded back, popping the extremely small shield you just earned every time you try to do it. It makes her passive feel like a complete trap.
2: Yuumi's lane presence is extremely poor. She can't threaten with autos because she can't get in range and her Q is lackluster damage, and has a windup for a rather weak slow CC. She has no other damage spells or CC, and her heal is weak at low ranks. Also her mana costs are just... YIKES. Extremely yikes for the poor damage she's able to do with her single damaging spell.
I don't think Yuumi should be a lane bully but at least give her even AA range with Nami, Sona, and Soraka. That would make it fair.