The best picks out of lane to counter Mordekaiser include Zeri, Aphelios, and Caitlyn. These champs are all great at countering Mordekaiser with decent win rates over 54%. Even if these Mordekaiser counter picks are not played opposite your position, it's not smart to take him into these matchups. They will likely crush Mordekaiser.
Conversely, if the opposite team choses either Lillia or Viktor, you should strongly think about choosing him, as he easily beats these champions with win rates over 53% against both ofthem.