How do I turn off Hide My Email on my iPad?

You can deactivate and cease using all the alias emails you have set up in Hide My Email, but there is not setting to turn off the service.

See Create and manage Hide My Email addresses in Settings on iPad - Apple Support

You can create and manage Hide My Email addresses in Settings. Go to Settings > [your name] > iCloud > Hide My Email, then...

• Deactivate a Hide My Email address: Tap an address (below Create New Address), then tap Deactivate Email Address. After you deactivate the address, it no longer forwards emails to you.