The Mystery Mushroom is a power-up that is new to Super Mario Maker that is only in Super Mario Bros. style. It was created so that every style has a style-specific power-up.
How it worksEdit
When you tap a mystery mushroom on-screen, it will show a menu to select from various costumes. A few are unlocked from the start. If you do not select anything, or select "mystery mushroom", the mushroom will give Mario a random costume. If a mystery mushroom has a selected costume, it will look like that costume in the editor. You can unlock new costumes by tapping in an amiibo to get a costume of that amiibo. You can also get mystery mushroom costumes by completing Event Courses or as rewards for the 100 Mario Challenge.
Note: You can only get the 30th Anniversary Mario costume via amiibo.