Make sure you’re never near a wall otherwise Poppy will be able to use her E and push you into it.
Whenever Poppy throws her Buckler and it lands near you, try to step on it. However, do not over commit to destroying it as she may use your positioning to play aggressive.
Post 6 it’s going to be difficult for you to kill Poppy as she will probably use her Ultimate R to knock you away. Keep this in mind and hold on to any CC abilities until she starts the channel.
Avoid fighting around objectives as Poppy may use her Ultimate R to knock away the Jungler and cost your team the Baron/ Dragon.
In team fights, disengage and avoid team fighting in the jungle as it will allow Poppy to get a good knockup with her Ultimate R or pick off a key target with her E.
Fighting around Poppy’s Ultimate R is a good way of winning team fights. She will knock back the frontline and then engage on to the carries while the frontline is missing. Try to dodge her Ultimate R, or use CC abilities on her so her channel is interrupted.
Depending on her build path, Poppy’s power spikes will change as her AD build and tank build have different power spikes.
Once Poppy hits level 6 and unlocks her Ultimate R, she will have a lot more utility and be able able to disengage from a skirmish.
The longer the game goes, the tankier Poppy will become. Try and reduce her gold income by denying her farm in the early and mid-game.