Smartphones are powerful enough to run many console games. In many cases, though, games are built for use with gamepads — smartphones have the horsepower, but not the hardware, for games that require intricate controls.
Thus, in the case of "Mario Kart Tour," it's likely that the game's controls will be pared down significantly from what you do in "Mario Kart 8 Deluxe" on Nintendo Switch.
The core loop of "Mario Kart" is racing (accelerating, braking, drifting, and jumping) and fighting (using power-ups gathered during the race). It's possible that Nintendo will require players to hold down a virtual button on-screen for gas/brake/etc., but it's more likely that the game handles acceleration while players focus on steering, gathering power-ups, and taking down foes.
There's a good precedent to look at here: Nintendo's "Super Mario Run." The game is still essentially a 2D Mario experience. You're running, jumping, punching question mark blocks and stomping on Goombas. But instead of having to careful maneuver Mario left and right, the game automatically handles Mario's running. It's not a perfect facsimile of traditional Mario, but it's pretty close.