You can print Legos - but it's not really a good reason to buy one. They're very slow to print due to the precision required and the materials that work best for them are all hard to print
That being said, there are tons of printable projects that scratch the same itch! Printable had a contest just last month for card kit models!
I agree that it'd be nice to have options for these things, but personally I like most of the features you want gone 🤷🏼♂️ for me - MK isnt for competitive racing, it's for chaotic fun - the handicap mechanics are great because it means you can play with anyone and still have a fun time. The person in last dropping a blue shell or bullet bill only spoils the fun if you're treating it like an esport, otherwise it just keeps things interesting
And as the best MK player in my circle, I can also attest that skill 110% matters in MK. sure you may get the occasional game where you just get stupid unlucky and your noob friend gets stupid lucky so they win, but 9/10 the best players still finish first in my experience