the UV light and time has made the plastic fall apart.
short of replacement with a trim panel in better shape there is not much you can do to make it look OEM.
the aftermarket reproduction quarter trim panels are complete garbage only good for a drag racer that needs to look like it has an interior.
1) replace it with a good original, very expensive, very rare.
2) you could smooth the panel, then either spray it the interior color you need, or buy textured winkle paint, spray it on let it wrinkle to kind of look like camera case finish then spray the correct interior color on top.
3) take the panels to a upholstery shop, they can make a camera case cover for the plastic panel like the older mustangs had, again expensive but will look much nicer.