Apparently there is only 1 level that no one (except the creator) has beaten. Its only 10-20 seconds long, but is so hard that its not clear if anyone will complete it before the servers shut down: https://www.youtube.com/watch?v=KmikpEVCuZE
My understanding of the level submission process is that you send Nintendo the level data and the inputs required to complete it. So theoretically, if you cheat by injecting artificial input data, you could submit a level that's effectively "TAS only".
