Ignoring the sales from their home country... that's a bad business model.
I didn't say they should ignore it outright but they certainly shouldn't make decisions that affect the North American market based entirely on what's going on with the Japanese market. They do that. A lot of Japanese videogame companies do that.
Why should Nintendo care for example if 3D Mario isn't as popular in Japan as 2D Mario is? 3D Mario is very successful in the larger North American market. Let's say they tinker with the next 3D Mario to make it appeal more to the Japanese market but these changes hurt the game's appeal in the North American market. So they trade 5 million American sales for 1 million Japanese. That's fucking dumb.
But Nintendo gets all uppity that Metroid for example is successful in the larger North American market but the Japanese don't like it so much. Since this is all about money that shouldn't matter. The larger market likes the product. If only one of the markets is going to embrace it the ideal one did.
Having both a 2D and a 3D Mario game for the Wii is ideal anyway since it covers all the bases and most Mario fans will embrace both.