You will not be required to create a Mii when you purchase the Nintendo Switch. Nintendo Life reports that the option will still be available and that more customization will be available for the characters, but Mii's will no longer be the mascots of the Nintendo brand. Looking at the quote from the company, it sounds like their Miis will be similar in status to the Avatars on Xbox One...

There is a Mii Maker on Nintendo Switch. The software to create Mii characters is now located in System Settings, and no longer resides on the HOME menu as a standalone application. You can also create a Mii character from the Profile screen. Making Mii characters is similar to the method used on past systems.

Mii characters can be used to represent a user profile, but are not required. Users also have the option to choose a profile picture from an included library of Nintendo character images. Mii characters can still be used in games if developers choose to include them.

You can send/receive Mii characters through local wireless communication between Nintendo Switch systems, or use an amiibo figure to transfer Mii Characters from Wii U and Nintendo 3DS systems.
Mii characters on the Nintendo Switch system are similar to past iterations, but they have some different options. More facial components and colors for hair and skin have been added, while the following attributes have been removed: creator's name, favorites, birthday, copying, and "public."

Personally, there's no love lost on my end. As cool as it was to see a cartoon character that vaguely resembled me in Nintendo games, it was never something I was that excited to see. The only time I think I really used it was for NintendoLand and that was just to help distinguish who was what character while I played with friends. Are you sad to see the feature go?