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.