From the Super Mario Wiki
Mushroomy Kingdom is a stage that appears in Super Smash Bros. Brawl and returns in Super Smash Bros. for Nintendo 3DS and Super Smash Bros. Ultimate. It is based on Super Mario Bros., but with a more barren and realistic look, rather than the typical 8-bit graphics used in previous Mushroom Kingdom stages. The stage layouts are almost exact replicas of Worlds 1-1 and 1-2 from Super Mario Bros., with the stage gently scrolling by. Due to this, players have to keep moving with the stage, as well as KO their opponents. There are many blocks, pits and walls to act as obstacles along the way. Other than this, there is no real danger, as the stage scrolls by slowly. Though there are two levels to fight on, the same conditions apply, as only the stage appears differently.
Several small changes were made to facilitate Super Smash Bros. gameplay: the Goombas, Koopa Troopas, Piranha Plants, Coins, and World 1-2's Warp Zone were removed; the Warp Pipes that led to bonus rooms and the flagpole of World 1-2 were also made inaccessible. Blocks that held Coins and power-ups were changed to release items in the stage, and Coin Blocks now offer up to three items. In World 1-1, the Hidden Block was lowered by a block, the flagpole and fortress are in the background, and the block the flagpole stood on was removed. In World 1-2, the Brick Blocks on the ground after the pipes and at the end of the level were changed to ground blocks in the stage.
In Super Smash Bros. for Nintendo 3DS, the World 1-2 version is removed, though the underground theme can still be played. The Final Destination form of Mushroomy Kingdom is set on the ground with two rows of Hard Blocks on top of it, creating a perfectly even platform, and the fortress can be seen in the background. Also, the unlock match for Dr. Mario takes place here.
In Super Smash Bros. Ultimate, the stage's graphics look more like they did in Super Mario Bros., while retaining the deserted and realistic look. The Hard Block the flagpole stood on is also present, though it is also in the background. Both the Final Destination and Battlefield forms of the stage are set on the ground, with Hard Blocks instead forming the pass-through platforms of the latter form, and the fortress and flagpole (including its Hard Block) are in the background. As in Super Smash Bros. for Nintendo 3DS, the World 1-2 version is absent.
Here is a chart for which buttons on which controllers to hold down to ensure which level will load up when selecting Mushroomy Kingdom (in Super Smash Bros. Brawl only):
Super Smash Bros. Brawl
Super Smash Bros. for Nintendo 3DS
Names in other languages