To unlock Toon Link first go to Subspace Emissary, make sure you've completed it, if not you will not be able to unlock him until you have done so. Go to Subspace Emissary and click on the completed data with the crown and load it. Next go to "The Forest" and choose any difficulty and any characters. Now go forward and fight whatever you by until you reach a door and enter it. Now you'll see Toon Link and fight him and win. After winning you'll unlock him.