Mario Kart has a large and flourishing competitive scene[1][2], including numerous competitions with prize money. In 10 days, Nintendo is hosting the Mario Kart World Championships where they are flying in the best players across the world to Tokyo for it. Obviously, not on the same level as F1 or CS, but there absolutely is a competitive scene for this with dedicated players.
For time trials, yes that is true. There are also other concerns such as that the speed stat is actually comprised of 4 different stats that have different values depending on the terrain: Ground, Air, Water, and Anti-Gravity. Some tracks that have a lot of water and so for that you would be better going for a kart that has high water speed. There are other less important statistics at play too that aren't mentioned here such as handling, traction, and also the hitbox of the vehicle is also important since it might change how tightly you can hug a turn against a wall or how wide you have to steer to collect coins.
In practice when playing online however, you won't know what track is about to be played, and so the meta right now prioritises mini-turbo stat much higher than speed. Having a high mini-turbo can also overcome the lack of speed by performing additional mini-turbos even on straight sections. Also when playing online you also will be hit by items a lot and need to try dodge other items being thrown at you, and for that having higher acceleration helps too.
[1] https://www.mariokartcentral.com/ [2] https://www.mk8dx-lounge.com/