Like why Mario and Peach play tennis and race go-karts with Bowser. Like seriously, what's up with that?
In an interview with GameInformer, Mario's creator dishes on very important things such as this. Here are some highlights:
Time and again, Bowser kidnaps Peach. Why do Mario and Peach still race go-karts and play tennis with him?
Miyamoto: If you're familiar with things like Popeye and some of the old comic characters, you would oftentimes see this cast of characters that takes on different roles depending on the comic or cartoon. They might be businessman in one [cartoon] or a pirate in another. Depending on the story that was being told, they would change roles. So, to a certain degree, I look at our characters in a similar way and feel that they can take on different roles in different games. It's more like they're one big family, or maybe a troupe of actors.
In Super Mario Bros. 3, the Koopalings were supposed to be Bowser's children. But there's also Bowser Jr. Are they all his kids, and are they all from different mothers? Is Bowser Jr. a Koopaling?
Miyamoto: Our current story is that the seven Koopalings are not Bowser's children. Bowser's only child is Bowser Jr., and we do not know who the mother is.
Mario's mustache and eyebrows are black, but his hair is brown. Does he dye his hair, is that natural, or is that a toupee?
Miyamoto: This goes into [a] technical explanation of the number of colors that you could use in the early days. Mario was originally red, blue, white, and black. Maybe it was around the time of Super Mario Bros. 3 that we changed his hair to a different color. What really happened was that, when drawing the character, it became much easier to draw the mustache as an extension of the outline of his nose. Using that same color of black, we drew the mustache. But then we felt bad for Mario that he didn't have a distinct hair color, so we gave him brown hair.
So Mario has naturally brown hair, the characters are akin to a troupe of actors, and Nintendo has no clue who bonked Bowser to make Bowser Jr.? Got it.
There's much more in the link below, so do have a look.
Mario's Creators Answer Burning Questions About The Series [GameInformer via VG247]