Downloadable content has seemingly been confirmed for Mario Golf: World Tour via the game's eShop description.

When originally posted, the description stated:

Test your skills in a gauntlet of course challenges, play with friends locally (using emoticons to cheer them on), or take on players around the world in online real-time tournaments. Discover additional paid downloadable courses and participate in special trial tournaments.

However, the last sentence was later removed from the description. Perhaps that information was prematurely revealed, but at least we know that downloadable content is indeed in mind for World Tour.