Nina is a Mii from the Switch.
Nina originally appeared as the Maker Mii for the 5 official Nintendo accounts in Super Mario Maker 2: NintendoUS, NintendoJP, NintendoEU, NintendoAU, and NintendoNM. She hasn't appeared in any game since. Obtaining her Mii data file is possible via downloading a course from one of the Nintendo accounts and extracting the save file. From there, decompressing the later.dat file in the save folder will contain her Mii data file.
Her Mii name in her Mii data file is ニナ , which simply translates to "Nina". Her height is set to 83 and her build is set to 40. Her Mii ID is as follows: 5B C1 3D EB 0A 46 43 ED A2 4E 5E 54 F2 F4 40 27
Her Mii data file contains unused Mii data. Her mustache is set to be 4 larger in size (the largest) and moved vertically 6 down (the lowest). This may hint at a possible earlier design for Nina. Additionally, her favorite color is yellow, which can not be seen in Super Mario Maker 2. In the game, she wears the default red Nintendo shirt, thus making her favorite color go unused.
Nina breaks rule 3.1.1.b in the Mii face rules laid out in the Guide to Using Official Accounts in Miiverse where it explains official Miis must not use parts in ways other than the intended function. In Nina's case, she's using eyebrows to represent strands of hair poking up from the top of her head.
Nina has a regular human design as well (left), similar to Hayley (My Pokémon Ranch). This is how she appears throughout most of the game. She appears in this design in Yamamura's Dojo, with Yamamura being a small bird character returning from the first Super Mario Maker game. She pops up generally around many tutorial-like or helpful menu options, as she is meant to help new players understand the game better.
However, as all Makers in the game are represented by a Mii, the official Nintendo accounts were no exception, and thus, needed a Mii to be represented. Nintendo decided to go with a Nina Mii to represent all five accounts.
It seems clear that Nintendo of Japan (most likely, the Super Mario Maker 2 developers) made the Mii, and sent the Mii to the other four divisions of Nintendo to be used for the other four accounts. Since Maker names and Mii names are separate, it shows that Nina's Mii data file in all five accounts are identical and have the Mii name still in Japanese.
Outside of her human artwork, Nina does not appear in any renders. Below is a screenshot of Super Mario Maker 2's Yamamura's Dojo > Maker Lessons > Intermediate > Treating the Player Fairly.