bond angle order decreases down the group 15 hydrides due to increase in p orbital character in sp3 hybridisation ?

The bond angle decreases down the group 15 due to increase in p orbital (which is another way of saying elctronegativity is decreasing )
Remember ‘s’ character increases electronegativity and ‘p’ character decreases electronegativity.

You can understand by this way:-
Bond angle: NH3 (107.8°) > PH3 (99.5°) > AsH3 (91.8°) ≈ SbH3 (91.3°) > BiH3 (90°) Electronegativity of N is highest. Therefore, the lone pairs will be towards nitrogen and hence more repulsion between bond pairs. Therefore bond angle is the highest. After nitrogen, the electronegativity decreases down the group.