The rudder angle on my boat is 90' when fully turned either way. Is that normal to be such an angle?

No not normal. Web searching throws up 35degrees each side as maximum - above this the rudder will probably stall. Would stall and cavitate at lower angles at the higher speeds. This 35 sounds about right. If you wanted to put stops on the tiller at 45degrees each side this would allow for occasions of slow speed manoeuvring in tight spaces possibly assisted by a kick of power to the prop to wash over the rudder.

And when reversing, you could not hold the rudder's force if it wanted to go to 90 degrees. So, some sort of stops or limits to its travel are needed.
Going forward, you'd never need more than 35 degrees. Car front wheels never do 90 degrees to the axle.

Thanks everyone. I'll put in some stops!