Really, if I had to choose (1 slot, a different plant, or doom shroom) I would pick the other plant.⠀ Doom shroom has a wide area of effect but leaves a 3 minutes crater. That really could limit your defense late game.
Plus, the cooldown is very long, even with imitater. All in all, it is mostly a better option to choose a different plant over Doom shroom.

Really, if I had to choose (1 slot, a different plant, or Doom Shroom) I would pick the other plant.⠀

The Doom shroom has a wide area of effect, but leaves a crater on the ground for 3 minutes. That could really limit your defense late-game. Plus, the cooldown is very long, even with the Imitater.

All in all, it is mostly a better option to choose a different plant over Doom-Shroom.

Really, if I had to choose (1 slot, a different plant, or doom shroom) I would pick the other plant. Doom shroom is good if you have open space, but that cuts into your available defense area. Nope, it is not worth it in the long run.

Really, if I had to choose (1 slot, a different plant, or doom shroom) I would pick the other plant.⠀ Doom shroom has a wide area of effect but leaves a 3 minutes crater. That really could limit your defense late game. 
Plus, the cooldown is very long, even with imitater. All in all, it is mostly a better option to choose a different plant over Doom shroom.