I don't think there's a problem with using pronouns (such as Okaa-san) if you spell them right, which this person seems to not be able too. Aside from the pronouns, the use of Japanese does seem random I agree. However, as a person who does use Japanese in her fanfic, I have to say that sometimes English is not effective in conveying cultural nuances in Japanese. Then again, I'm quite comfortable in Japanese, so stuff like that doesn't bother me as much as someone who doesn't speak it as well.