As an American born white chick, as a culture Americans IMO are pretty ignorant of other cultures... there are always exceptions but we are pretty ignorant.
I know my T struggles to understand that despite being very Caucasian and having a Dutch surname my grandfather was Muslim. T has made some pretty crazy assumptions just based on religion. So we tend to revisit it each time my grandfather comes up in therapy. I’m OK raising T’s awareness of this as it doesn’t really impact my therapy and enjoy helping open peoples minds when they are willing. I understand how it could feel like a burden too, especially for you as your T’s misconceptions are more directly impacting you and the therapeutic relationship. Depending on where you are in the US it may be difficult to find someone well versed in diversity or who shares your culture. My experience has been that it was easier on the coasts than it has been for me in the Midwest... but then those are MY sweeping generalizations and assumptions.
If you feel comfortable I would confront the generalizations and maybe spend some time talking about your culture. I would also mention how the assumptions make you feel if you are comfortable sharing that as some people are not aware of how hurtful assumptions and generalizations can be.