If I remember correctly to 100% Mario Odyssey you need 999 moons. The game gives you 850 or so normal/good/fun moons, but the last 150 or so you have to buy with coins. For me this meant grinding a flower challenge in Bowsers Castle ~20 times in a row.

I did it, but that part wasn’t fun.

The rest of the game is perfect, but that part would keep me from 100 percenting the game again.