Why does Shiina always say "how shallow-minded"? She usually says the phrase "how shallow-minded" (あさはかなり,asahaka nari). Is the fact that she came from the past (a.e Feudal Japan) and was born in Ancient Japan has something to do with it?
As stated in the wiki page you linked to, it's most likely because she's from an earlier era.

Also, being a trained assassin, she most likely views the actions of other characters as childish and foolish, so she just gives her honest opinion on the matter.