Poppy is kind of underwhelming.
She looked really fun in all the videos and the spotlight and all that, and she's adorable as hell... but that's really all she has going for her. Her damage is pathetic, so you may as well not build any AD at all, and all she can really do is use R, since no one ever gets near enough a wall to use your E (and even if you do, what'll happen? You can't kill anyone fast enough to make use of that stun).
Sure, you can throw someone halfway across the map with R, but half the time that doesn't really do anything, since you essentially do nothing in teamfights, it's still a 4v4 anyway.
Illaoi is weak too (44% winrate, hey there), but at least she's capable of doing damage and making a difference if people misplay against her. Poppy? You're better off having an Ali supp or Braum kill someone --- in fact, that's what Poppy is now; she's just a top lane tank supp.
I donno. She just doesn't seem very useful at all. Other tanks just hand her ass to her with 4x the damage (Mao absolutely dominated lane against my Poppy today), and she just does no damage at all, even when I built her full damage. She needs some tweaks to actually let her contribute in a fight other than pushing R, and she needs to be able to actually trade with someone and not horribly lose.
Probably the only person that feels Poppy is kind of underwhelming (same with Illaoi), but whatever. Even unpopular opinions are valuable.