I think you’re both underestimating and weakening the card.
1) You don’t play zombies. How would you know anything about the play-rate of -cuke? I can tell you that it has been played and is being played.
2) The difference between -cuke and Final Mission is enormous. That is not to say one is better than the other... they’re just not really in the same category. Mission is kind of like a Goblin Grenade. Cuke is kind of like a Wrath of God. They have very different purposes and belong in different sorts of decks.
3) The most tactical way to play -Cuke (see what I did there?) is to do it when Zombies are clogging the ground lanes (whether or not there are plants) and/or when you have sun left over to plop a plant in a now-empty lane for face damage. Making those lanes unplayable takes some of the tactical out, and weakens the card overall late game.
4) I agree with you about modal cards. Having some (both sides) could be cool. Unfortunately, I can’t think of any cards yet printed allow you to choose between modes when played, plants or zombies. I don’t think modes are really in the cards (see what I did there?). Maybe a card such as -Cuke could read “If there are no Zombies on the ground, kick the Zombie player in the face for X damage,” but I doubt you can swing “Choose one: Destroy all ... OR deal X damage.”