Keep it in the hands of the pro's...... a racer needs a certain amount of though put into the track and fairness of it.
What makes Mario Maker work is that its a platformer, and you can scale the difficulty with it.
That does not apply to Mario Kart.
It would just be sub par raceing tracks, and less fun races overall.
IMO this is a stupid idea, and I cant think why anyone voted yes.