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:
In my opinion, the most lethal Poison Mushroom is the one with a skull on its head.
Power ups in the Mario franchise usually have good effects, some make you bigger, invincible, able to fly...
Poison 'Shrooms on the other side always 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 could easily represent 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.