User blog comment:TheHandsomePlant/(SPOILERS) Full leveling data of all plants (5.7)/@comment-24951189-20161225072258

I think the reason that Shadow Shroom's duration decreases is that the poison is supposedly killing the zombies faster. The total damage still increases every level. It's meant to finish off zombies before they hurt your plants, without being OP (so to speak) against super-healthy zombies.