If your iPad or iPhone screen is too dark even though you have set the brightness to 100 percent, do this:
Go to Settings > Accessibility > Zoom.
Check that zoom isn't activated if you don't need it, or...
If you need zoom, check that Zoom Filter is set to None.
This often solves the issue with an inexplicably dark display.
You can check and change brightness settings in Control Center or Settings > Display & Brightness.