The poison mushroom is inconsistent in design and effects between Mario games. I took the Mario Party 3 design which acts as a “curse,” and it makes you go slower: Link
In my opinion, the most lethal Poison Mushroom is the one with a skull on its head.
Almost every power up in the Mario franchise has a good effect, it makes you bigger, stronger, faster…
Poison ‘Shrooms have negative effects. If you’re bigger, it makes you smaller. If you’re already small, it kills you. In Super Mario RPG, it drains your health. It represents the hurtful things in your life that beat you down. The opposite of a power up, a toxic emotion in your life that drains your happiness away.