Bowsette is a fan hybrid character of Princess Peach and Bowser, because gender-bent characters is one of the foundational rules of the internet.
The discussion about whether or not Bowsette should be canon was ignited up when fans discovered that Toadette and Peach can combine with the Super Crown item in New Super Mario Bros. U Deluxe to create the "super-pwered Peachette."
Because if Peach and Toadette can combine into a new character, shouldn't Bowser and Peach be able to do the same?
Obviously, Nintendo disagrees.
Via ResetEra, the company posted on the New Super Mario Bros. U Deluxe website that only Toadette can use the Super Crown item, thus Bowsette is not a possible configuration. Nor is a Luigi-Peach hybrid, since it is Nintendo canon that Luigi likes dressing up as the princess.
Now, Bowsette has pre-dated the Super Crown in New Super Mario Bros. U Deluxe, but her possible canon origins are brought up every time there's a new Mario game. For Super Mario Odyssey, fans created origin stories for Bowsette that involved Bowser throwing his version of Cappy at Peach to create the hybrid character. Bowsette owes her existence to the art of Ayyk92 who created a comic strip in which Bowser obtains a power-up item that turns him into woman who much resembles Princess Peach. The comic was released on September 19, 2018.
Naturally, news of the Super Crown item sparked another Bowsette comic from the artist.
The Super Crown's some spicy new Mario lore pic.twitter.com/7DQe6UXvLQ
— haniwa (@ayyk92) September 19, 2018
For now, however, the official description of the Super Crown will remain, "When Toadette finds one of these, she can transform into the Super-powered Peachette. (Sorry Luigi-only Toadette can use this item!)"