I would we use a two tailed test for a testing mortgage interest, aren’t mortgages have to be one sided because they can’t be negative?
A two tailed test is used to research deviations from the null hypothesis to the upside or the downside.
So if you want to test the null hypothesis that the average mortgage rate across all mortgages is equal to 5% a two tailed test will allow you to deduce whether or not the rate deviates significantly to the upside or to the downside from 5%.
If you are interested if the average mortgage rate is significantly higher than 5% you would use a one tailed test (with the null hypothesis that the average mortgage rate <= 5%).
Aside: Why can’t mortgage rates be negative? 