Splatoon has been a major success for Nintendo in all areas of the world, even moreso when you consider the small install base of the Wii U. Perhaps the sales scene in Japan goes to show just how well the game was embraced.
When you look at sales numbers, Splatoon has sold 1.356.993 in Japan. If you don't count the 'Wii' series of games, that makes Splatoon the most successful IP launch on consoles when looking at previous generations (PS2, PS3, Wii). The tie ratio for Splatoon is just about 50%, meaning half the Wii U owners in Japan own a copy of Splatoon. I'd say that's a pretty impressive stat!