Madshroom is one of the three skins released in The Fungus Amongus Planet event. It could be purchased in the item deal part of that event in early April 2020.
Madshroom could be purchased for $4.99 in a limited time deal that included a package of 1400 DNA. It was not available for individual purchase and may not be available for purchase in the future.
Details of this skin may vary depending on location, platform or time.
Note: Madshroom was released again for a limited time in the Best Skins of 2020 offer in late December 2020. For this release Madshroom could be purchased in an item deal for $14.99. The included items were The Cheshire skin, 25000 coins and 2000 DNA.