I've run through all of 50cc Using various characters getting 3 stars in each cup. There is a sticker for Miiverse associated to finishing a grand prix with each character so I am working through that just now.

100cc is a lot harder. The AI is a massive pain in the ass in this game, and they removed the Bikes with the sweet turning that were in Mario Kart Wii. Man I'd like to see them again.