You name all those games and that’s wonderful but it’s not on par with Splatoon 2, Mario: Odyssey, BotW, MK8D, Mario & Rabbids and Xenoblade Chronicles 2. Their first year was success after success, but their second was just stagnant overall with them sitting still after achieving a powerful start for a new system. An incredible December release (Smash) doesn’t make the other 11 months look any better.
Mario Tennis Aces is a good game, but not a spectacular one. Same goes for Octopath and Mario Party, but please don’t use DLC and a bad mobile port as a system seller to prove the year was consistently good. No one is saying Nintendo didn’t release good games, but that their lineup is not the sort of quality we expect from them. What you listed is not a strong mid-year lineup by any metric.
Compare what you listed with Bayonetta 3, Animal Crossing, Luigi’s mansion, Core series Pokemon, Daemon X Machina, No More Heroes, Town, Yoshi, Metroid Prime 4 and whatever 3rd parties are on course to release and then tell me 2018 was strong. If every year is strong then the word loses all meaning.