Japanese characters are hugely represented in a game from a Japanese director for a Japanese company...wow. Crazy stuff is happening. Also I think you aren't objectively looking at the cast, you just dislike FE and use their amount of characters to justify your argument (if you don't like FE, I am not going to argue about it cause to each their own). It's actually pretty even for characters that are Japan to not Japan.
The problem is it's easy to say "oh, character X is japan" cause it's super subjective for a lot of characters. But basically all Star Fox, DK, Mario, Pokemon, Zelda, Metroid characters aren't japanese. FE is, Kid Icarus is, FF is, SF is, Shulk is, and who else? That's like 40+ to 15 maybe. Seems balanced to me.
Edit: Forgot Earthbound for Japan.