Olaf

Olaf wins early fights 1 v 1 and 2 v 2. Later on poppy has no way to deal with olaf due to his R

Master Yi

Yi can easily avoid all your abilities with Q and shreds you later on into the game

Amumu

Amumu Q can be blocked by your W and can be ulted out of fights later on

Elise

Very strong early but can be dealt with later on.

Lee Sin

Q and W can be blocked, his mobility is not an issue.

Gragas

body slam is easily react able and is cancelled by your W, you outdamage him early and scale into tank later on.

Zac

Zac can never engage into or near poppy and loses early to her.

Kindred

you can win early skirmishes against her if played correctly but get outscaled later on

Dr. Mundo

win early lose late

Ekko

out damage him early if you block his E, later on can be peeled in team fights

Graves

lose fights early on due to innate tankiness, he outheals your damage later on but can be disrupted in teamfights.

Jarvan IV

gets out damaged early, and has no mobility against poppy due to W

Jax

can be beat early on but gets out scaled hard, in teamfights jax can be disrupted and peeled

Kayn

wins in early fights and can be dealt with in later stages of the game

Kha'Zix

only way for kha zix to be useful against poppy is if he snowballs early, you outdamage him early and outscale him

Poppy

Poppy

Rengar

Same as kha'zix but snowballs harder

Rek'Sai

Can beat you early on, you can cancel her tunnel with your W

Trundle

Is stronger than you at all stages of the game

Sejuani

Udyr

Hard to escape due to bear stance and out damages you throughout the game