The best champions outside of lane to counter Diana include Caitlyn, Aphelios, and Zeri. These are all great at countering Diana with decent win rates over 55%. Even if these Diana counter picks are not picked in the same position as you, it's not smart to play her into these matchups. They will often beat Diana.
Yet, if the enemy team picks either Karthus or Amumu, you should seriously consider selecting her, as she easily crushes these champs with win rates over 54% against both ofthem.