Unfortunately this isn't quite as simple as it might look. Default username policy for a Moodle site specifies that a username be only comprised of alphanumeric lowercase characters, underscores (_), hyphens (-), periods (.) or at symbols (@) alone, but by enabling the admin setting extendedusernamechars it's possible to set full-width usernames too (e.g. a user with the username ムードル博士).
In order to address the issue you've identified, we would need to detect whether extendedusernamechars is enabled. If it's disabled, we should perform a zenkaku->hankaku conversion on submitted usernames; if it's enabled, we should do nothing (or we'd stop users from being able to log in).
Will raise an issue for this once things are a little less busy here at HQ.