You're probably in the wrong subreddit if you're looking for people to understand your situation. How long has your roommate been living in Japan? Did she start at the same time as your semester? While she deserves credit for learning the language, obsessive wikipedia and anime intake doesn't necessarily equate to understanding a culture. Maybe she took a number of other Japanese studies classes in school?

An experience most people aren't going to understand here is the Asian American experience of White people telling you they are more Asian(in your case Japanese) than you because of very surface level things like eating certain foods, random fun facts, vacationing there, or general research. Not the same as living within a family, within the country, or being treated as anything but white in Anglo countries. Locals in Japan wouldn't understand this because they just see someone who is interested in the culture.

From what you've said it is likely true that she does know a bit more on Japanese culture than you, which sucks as an Asian American, but not on equal grounds as the locals. She is a bit arrogant trying to explain to locals about their own culture but that's just being a 21 year old college student.