The best picks out of lane to counter Dr. Mundo include Zeri, Tristana, and Caitlyn. These picks are all great at countering Dr. Mundo with win rates over 54%. Even if these Dr. Mundo counter picks are not played opposite your position, it's not a good idea to choose him into one of these matchups. They will likely beat Dr. Mundo.
Yet, if the enemy team brings either Galio or Rumble, Dr. Mundo mains should seriously consider taking him, as he easily dominates these champions with win rates over 55% against both ofthem.