Should frameless glass shower door hinges be screwed into the wall at an angle?
Any of you know if this is normal?

I would say not normal.
Looking closely at the photos i would say that if the manufacturer had wanted the screws to be at an angle then they would have set the countersinks in the brackets at that specified angle so the heads did not stick out proud of the bracket, They did not. The countersinks are straight so therefore one can assume that the screws SHOULD go in straight.
I would surmise that  the installer is feeding you a line to cover his mistake.