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.
