It appears The Legend of Zelda: Breath of the Wild uses an upgraded version of Nintendo's classic Mii character creator to design many of its NPCs.
Within Breath of the Wild's files, NPCs are referred to as "UMii"s (Ultimate Miis?), who use a combination of familiar Mii facial components for their various designs.
Breath of the Wild's versions of these components are more detailed, and stylised to fit the game's art style. But it is noticeable how many NPC characters share various bits and pieces of the same face parts.
This also means that, with a bit of code injection, it is possible to customise the game's UMii characters to display with different face pieces, and even get your own Mii in Breath of the Wild too, as HEYimHeroic shows:
Hi, Mii expert here. Turns out, the NPCs in TLoZ:BotW use an advanced version of the Mii format. This means that with modding, you can inject Miis into the game. :)— i'm alice (@HEYimHeroic) January 4, 2021
Thinking about opening commissions for Mii injects, both screenshot/images of your Mii and mod downloads! pic.twitter.com/8NfVr4zyqA
"UMiis have almost all the same parameters as Wii U/3DS Miis, with a few minor differences here and there, like moles no longer being supported," HEYimHeroic reported. Some hairstyles are also not included, with Breath of the Wild instead converting outliers into a list of more Zelda-friendly designs.
UMiis represent the bulk of the game's NPCs from both human and hylian races - alas, there's no way to turn yourself into a goron.
You also can't use this method to edit the designs of main characters, whose character models have been created individually - so you'll just have to stick to dressing up or undressing Link as standard.