How effective is pseudoephedrine in fixing ear pressure blockage during flights?

It may work. It may work.. It just takes a while and you may not need it:try performing a valsalva maneuver - pinch your nose and gently "blow" air out of your nose while at the same time bearing down like you are lifting something. This will force air up your eustachian tube and expand the middle ear and prevent traction on the ear drum which is what causes the pain and fullness during descent.
Not so much. Pseudoephedrine can work if the reason you are prone to blockage is allergy. Pseudoephedrine is an antihistamine that improves symptoms of allergy. If you aren't having allergies, it is not likely to help with ear pressure during flights. Try chewing gum or sipping water. The swallowing usually opens the eustachian tube and equalizes the ear pressure with the surrounding air pressure.