I don't have the Wii U version, but here are 1080p Switch captures without any JPG compression/artifacts or subpar blurry resizing like the examples being passed around.
4K versions for comparison's sake (2x resolution with bicubic filtering to mimic a typical HDTV's slightly blurry upscale processing)
http://i.imgur.com/lVu5Jnb.jpg
http://i.imgur.com/srD627W.jpg
http://i.imgur.com/sgN4EX3.jpg
http://i.imgur.com/184Y3PY.jpg
4K versions for comparison's sake (2x resolution with bicubic filtering to mimic a typical HDTV's slightly blurry upscale processing)
http://i.imgur.com/lVu5Jnb.jpg
http://i.imgur.com/srD627W.jpg
http://i.imgur.com/sgN4EX3.jpg
http://i.imgur.com/184Y3PY.jpg