That's a bit of an overreaction based off of one game that everyone can agree was a flop. And even then, Ultra Smash is only a flop in the sense that it doesn't have the content to justify being sold at full price and they have not promised regular content updates (a la Splatoon). Otherwise, the game has been praised for still having really satisfying and surprisingly deep tennis mechanics, as well as great visuals. It's just that the game feels more like "a solid, €20 game from an upcoming and talented indie studio", rather than a €60 Nintendo licensed game.
I hear that Camelot's B-Team handles the Tennis games though, while their A-Team handle the Golf games. So comparing the Tennis and Golf games isn't even a fair comparison as the A-Team has far more to work with.
I honestly don't play the Mario Sports games but they are the sort of game where it's really easy to see if they are worth it or not, and in the case of World Tour, it's definitely a quality title. I hear it has twice the content the Gamecube one had, and that's BEFORE you get any DLC for it. And you could even customise your Mii and stuff in it like in Smash.
Oh, and the celebration animations are by far some of the best things to come out of the Mario series in recent years. So much love and care put into them.
Upon an Eagle, or a Hole in One:
-Mario commits murder
-Waluigi becomes fabulous
-Birdo snogs the camera
-Nabbit runs across a NSMBU level with a HUD and everything
-Rosalina tears time and space asunder with actual identifiable galaxies from the Galaxy games in the background, and summons almost every variation of a Luma from the Galaxy games to her side