I'm actaully finding it fine designing on tv with the Pro Controller, but there's some fiddiliness yeah. I definitely remember finding it easier on the Wii U Gamepad (BRING BACK THE WII U) with the stylus.
UPDATE: Happy national videogame day! I got a stylus and yeah there's no chance I'll ever design with the controller again. I made a level that I think is better than the other two. It took me a lot longer to make, anyway. It's called "Lakitu's Castle In The Sky".