It seems a Japanese stereotype is true - at least to me. I know several people who came from Japan, and they are amazing at MKW. Just pro, doing expert shortcuts with ease. However they've been playing since the game came out in Japan, I'm not sure every Japanese person is this pro xD
I think the Brits are pretty good, but so are the Americans. Remember, there are only a few truly amazing Mario Karters out there, the rest are not quite as good, but still good in their own ways. I think since America has a large population, we therefor have a larger percentage of better players. Same can be said for Brits, and the Japanese are the Japanese.
There's actually a MKW tournament every year called the "Tri-Country Championship" where some Americans, Brits, and Japanese play together (4v4v4) in a series of elimination matches. Its fun to watch.
Tl;dr -> Everyone is good at their own thing. Some just are better at video games than others.