well, considering it is the second best selling 3rd party game on a home console, only behind FFXIII, the sales don't seem that bad to me. I can see why people would expect more do to the PSP versions, but it outdid every PS2 version, so it is doing good.
Also, if passing 1m is only "ok" then all but 13 titles must have horrid sales in Japan (on home consoles)
As a side note, there have been 33 1m sellers on DS, and 3 1m sellers on PSP.