The top champs not in lane to counter Janna include Samira, Kalista, and Kog'Maw. These picks are all good at countering Janna with decent win rates over 59%. Even if these Janna counter picks are not chosen in the same position as you, it's not smart to take her into any of these matchups. They will very likely beat Janna.
Yet, if the opposing team choses either Rumble or Trundle, you should strongly think about selecting her, as she easily beats these champs with win rates over 55% against each ofthem.