Is doom shroom worth its price?

To offer a counter-perspective; it can be, depending on the situation.

Yes, during the main campaign, not really. You may find it useful in some levels, simply due to its cheap cost and effectiveness, but after a while it doesn't really pay off as you find other options, like the Cherry Bomb, or the Jalapeño, etc.

However... The Doom Shroom can act as a "back-up" Cherry Bomb, in tight situations. For example, on some survival levels you may get Gargantuar Zombies, and Enraged Gargantuar Zombies (sometimes even 2 waves in a row), which can really put pressure on your defenses. In those situations having a backup option to deal high damage in a 3x3 area definitely pays off, and dealing with an unusable square or two instead of having to rebuild half your defenses definitely makes them worthwhile.

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.