The top champs out of lane to counter Lillia include Zeri, Samira, and Kalista. These picks are all great at countering Lillia with decent win rates over 54%. Even if these Lillia counter picks are not picked opposite your position, it's not smart to take her into these matchups. They will very likely crush Lillia.
Conversely, if the enemy team brings either Heimerdinger or Akshan, you should strongly think about picking her, as she easily beats these champions with win rates over 55% against each ofthem.