The Mario Circuits don't really show elements of the courses belonging to Mario. But look at the course. They usually consist of rolling greens, warp pipes, Mario enemies, Peach's Castle... The Mario Circuits don't represent Mario, they represent the game series, since it includes many of its elements!
Being in space for presumably billions of years must make her pretty bored as well.
Some kart bodies give a speed boost in the air or underwater. Some are obvious, like the Soda Jet in the air or the Tiny Tug in the sea. It may seem odd that the Pipe Frame gives an underwater speed boost, but remember that water usually travels through pipes.
7's gold tires and gold kart give a boost in weight when used due to how dense gold is in real life.
It may be a bit unusual at first for an Arabian theme stage to deal with Shy Guys, but it has shout-outs to Super Mario Bros. 2, which was originally called Doki Doki Panic, which itself was an Arabian themed game. (And also the first true appearance of the Shy Guys.)
From DS onward, every Mario Kart has had 32 courses. The same as the number of levels in Super Mario Bros., and in the same arrangement (8 cups\worlds with 4 levels each). Granted, it could be unintentional, but still.
Dry Bones's high acceleration and handling make it easy for him to recover from damage. This goes along well with his status as a Reviving Enemy in the main series; knock him down and he always gets back up.
The Green and Red shells are parallel to how the Koopa Troopas function as enemies in 2D games. The Green Shell are the most likely to miss their target similar to how green Koopas can fall off cliffs while the Red Shell could be a parallel to how the Red Koopas are able to turn away before falling.
Whenever Koopa Troopa is a playable character. The Koopa still has a chance to get a shell, think about it for a second...
Granted, by the time the first Mario Kart game came out, it had already been established in Super Mario World that, unlike real turtles, Koopa shells are detachable and their owners can survive without them. Though that still doesn't answer where all those empty shells come from...
The original Paper Mario had a Koopa saying something about how he had his shell custom-made and shipped from somewhere else, so maybe Koopa shells aren't even part of their anatomy. Or they are, but these are fake.
Despite real-life turtles having irremovable shells, nothing says that Koopa Troopas simply can't take off the shells that they are born with.
You can throw/set Bob-ombs in Sunshine Airport in 8. That's right. You can use explosives in an airport. It's a TSA agent's nightmare.