Mari Monshroom is one of the four skins released in The Fungus Amongus Planet event. It could be purchased in the item deal part of that event in mid January 2021.
Mari Monshroom could be purchased for $4.99 in a limited time deal which included packages of 8500 coins and 1000 mushrooms. 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.