Nintendo has confirmed that tomorrow morning (April 23) Mario Kart 8's servers will go offline for a few hours for maintenance (BST 8:50am-10am / CET 9:50am-11am).
This downtime is planned so they can improve performance for the racer before DLC Pack 2 and the new free update are released - also on April 23. This new update will introduce the long awaited 200cc class, as well as the second downloadable content pack that includes new courses and returning favourites from older games.
Thanks, Nintendo Everything.