Mushroomy Kingdom is a stage in Super Smash Bros. Brawl and the 3DS version of Super Smash Bros. for Nintendo 3DS/Wii U. The stage is an exact replica of World 1-1 of Super Mario Bros., though apparently age has come into play and the entire area has become a desert with plenty of ruins. The stage will slowly scroll right, meaning you'll have to attack your opponents and continue moving during the whole engagement. Once selected, you may also appear in World 1-2, the first underground segment of Super Mario Bros.
Ground Theme (Super Mario Bros.)
Ground Theme 2 (Super Mario Bros.)
Underground Theme (Super Mario Bros.)*
Underwater Theme (Super Mario Bros.)*
Underground Theme (Super Mario Land)*
The Mushroomy Kingdom stage is based on the first and second levels of the original Super Mario Bros., depending on which stage the player gets. It's actually random, though you'll either get the entire first stage or the entire second stage, though in the future when everything has turned to ruins, hence the name "mushroomy", meaning it's not quite the way it used to be, though similar nonetheless in the entire structure.
It seems as if the enemies that once infested these lands have moved on, or have all been exterminated by the Mario Bros! The "?" Blocks remain, however, and they'll also give the player items as well just as they did in the games. This practice has been in almost every game in the series, including the main platformers, the Paper Mario series and more.