A powerup that appears in New Super Mario Bros. U Deluxe that transforms Toadette into "Peachette", a Princess Peach lookalike that gives her the ability to Double Jump, and perform Peach's Floating Jump. Additionally, she automatically gets boosted if she falls into pits.
Among internet circles, however, the Super Crown is used to transform many more characters into human female Peach lookalikes, typically referred to as "peachifying" (ピーチ姫化), or as a personification item in general. They may additionally be referred to with either an "-ette" suffix in English (in reference to Peachette) or by default a "princess" title like Peach ("-Hime" suffix in Japanese).
(according to topic #15603 and OG Bowsette's wiki)