Nintendo is shaking up its online services with the Nintendo Switch, planning a paid service that will include access to lobbies and voice chat via a smart device app, for example. Part of the updated online system will evidently be a User ID, which will likely pop up in place of your account name when playing online. [Update: Apparently this has been there a little while, so consider this a 'reminder' if you prefer!]
You can now login to the Nintendo Account page and - under the 'User Information' page - set your User ID. This is a separate field that, when completed, ties into your account.
It's a good chance to setup your alias before it's snapped up; it'll be your alter-ego when splatting everyone in Splatoon 2, for example.
Have you setup your User ID yet?