The top picks outside of lane to counter Cassiopeia include Tristana, Samira, and Draven. These champs are all good at countering Cassiopeia with decent win rates over 56%. Even if these Cassiopeia counter picks are not used in the same position as you, it's not smart to play her into one of these matchups. They will very likely crush Cassiopeia.
Yet, if the opposite team brings either Ziggs or Wukong, you should strongly consider selecting her, as she easily crushes these champs with win rates over 54% against both ofthem.