Well, I'm sure they could set a mobile app up in a way where the levels could be played/designed on the Wii U as well. That way you could come up with basic ideas and such on-the-go and more thoroughly play and tweak them later on a proper system. Touchscreen controls, while far from ideal, are also good enough to get some idea of what works and doesn't work in a Mario level, though you sure wouldn't want to play a whole game that way.

If Super Mario Maker is here to stay, they are going to need to find a way to let us make levels on our phones and tablets eventually. The next system controller probably isn't going to have a touchscreen in it.