VGFacts

You might notice throughout the franchise, all but one non-original item (by non-original, I mean not made specifically for the Smash Bros. Franchise) has their design remaining true to their home franchises. The one except is the Poison Mushroom, which unlike in Mario Games, has been like Super Mushrooms, but not as red and have themselves meaner looks, done to confuse players.

Watchmojo.com are even able to explain that the confusion has put Poison Mushrooms on the list of Worst Smash Bros. Items.