What happens if I link a Nintendo Account and then decide I want to delete the Nintendo Account?

All Animal Crossing: Pocket Camp data (including Leaf Tickets and items you've purchased/obtained) up to the moment you delete your account will be deleted.
 
If you want to use Animal Crossing: Pocket Camp again, you will have to delete the game and reinstall it. You can then start using Animal Crossing: Pocket Camp from scratch.