Just before Miitomo was inevitably terminated, Nintendo announced that they would be launching a new browser-based feature that will allow Nintendo Account users to create and edit their Mii avatars online. Now, Nintendo have officially launched the feature and have called it Mii Studio.
Mii Studio allows you to create and edit you Mii just like you can on the Wii, 3DS, Wii U and Switch. You can choose gender, hair style and colour, height, eye colour and your favourite colour. You can sync your Mii with your Nintendo account and it will replace your Mii Avatar on My Nintendo but only if you want it to.
If you would like to try out Mii Studio for yourself, you can find it here.