Statistically, there’s nothing wrong with the null hypothesis being p=0.9. It’s probably not a test you would see very often in practice because usually there is nothing interesting about p=0.9. But if you wanted to test whether or not p=0.9 for some reason—any reason, setting the null hypothesis as p=0.9 is a perfectly valid (frequentist) way of doing it.
Statistically, there’s nothing wrong with the null hypothesis being p=0.9. It’s probably not a test you would see very often in practice because usually there is nothing interesting about p=0.9. But if you wanted to test whether or not p=0.9 for some reason—any reason, setting the null hypothesis as p=0.9 is a perfectly valid (frequentist) way of doing it.