It's reasonable for people in the US to know it. I do but then I've been there. But I'm not sure it's that much more basic knowledge than the capital of California, Texas, or New York if you don't live in those states.

Most people think the capital of New York is New York. Ottawa isn't exactly on anyone's world destination list. Completely reasonable that the majority of people wouldn't know that.

OK, I guess I underestimated how basic this information was. Now, I'm wondering what the interviewer is trying to prove by asking this question.

For Canadians? No. For the rest of the world? Much more likely.