iPad screen dims

Your iPad will always dim the backlight in certain circumstances. Your iPad's thermal management may be at work.

GPU/CPU intensive activities, or when the iPad is being used in very high ambient lighting conditions (such as when used outdoors, or in direct sunlight), the iPad may be overheating. Amongst other heat reducing strategies, your iPad will reduce screen brightness.

If your iPhone or iPad gets too hot or too cold - Apple Support

Your iPad display dims because of two possibilities:

1. If you have auto brightness turned on, it will do that when you switch lighting environments.
2. If you're on the same lighting environment and it's still dims, your device might be running too warm to the point where the supply has to dim temporarily to protect the hardware.