The best champions outside of lane to counter Poppy include Zeri, Lucian, and Draven. These picks are all good at countering Poppy with decent win rates over 58%. Even if these Poppy counter picks are not played opposite your position, it's not smart to play her into these matchups. They will often win against Poppy.
Yet, if the other team choses either Vel'Koz or Swain, Poppy mains should seriously think about picking her, as she easily crushes these champs with win rates over 54% against both ofthem.