There is a recipe with fly agaric that also counts as a mushroom.
And yes, its basically a triple piggy skill, I don't think going full mushroom is relevant late game.

If you have 2 mushrooms in your bags, you get 1x Fly Agaric every time you enter the shop after a battle.

It's just econ item if you got 2 shrooms by the time you get it. If not, don't bother with the item. +2 gold for selling extra shroom per turn is much better than many of the alternatives while battle effect is worthless even when you combine it with scythes and food amulets. If you wanna use it late game upgrade shroom or two into doomcaps.

One thing to remember about Doomcaps is every time they trigger they also lower healing by 10% and that stacks. So if you are running either poison or dark you are crippling many builds but you want to make sure you still have damage going high enough to compete at higher levels,