You can tell from the naming convention that "Mario Kart Tour" is its own thing, separate from the main thread of "Mario Kart" games that have come out on Nintendo consoles exclusively. The latest game in that series — "Mario Kart 8 Deluxe" — is the most recent main series entry. "Mario Kart Tour" is a side game, like "Mario Kart Arcade GP."
That's not a bad thing, necessarily.
Nintendo hasn't made a bad "Mario Kart" game. At worst, some entries in the series have felt obligatory rather than essential. That hasn't been the case in recent years, and there's no reason to suspect that "Mario Kart Tour" will be anything less than good.
That the game isn't a numbered entry isn't the only indication that it's a spin-off: The fact that it's for a smartphone, not one of Nintendo's own consoles, is another big indicator.
Nintendo's approach to smartphone gaming has been focused on crafting its franchises specifically for the platform, rather than trying to force its console games onto a mobile device. This results in spin-off games with controls and gameplay mechanics that only make sense on smartphones and other touch-based devices.