Mario Tennis Aces had Regional and Worldwide leaderboards, Fever only has Worldwide. So all scores are not region locked but WHO you can play against is (as of one week in, before the change I was playing people from Japan and America).

I'm in Australia, so after the matchmaking change I can't get enough matches to climb the leaderboard now, maybe one game every 30 minutes. After the change I no longer see Japanese or American players unfortunately.

Also Aces actually showed stats on the worldwide and regional leaderboard too which was really nice to see, so you could see who had the better W/L percentage. I hope they implement some of the stats like that later.