How do I remove activation lock on late mother's iPad?

First and foremost I am very sorry to hear about that and my condolences to your mom. Apple typically requires legal documentation to proceed in accessing a deceased person’s Apple Account. Once you do that and get access, you will be able to remove the Activation Lock.

Please review: How to request access to a deceased family member’s Apple Account - Apple Support

Hello~ First, please accept my sympathies for the loss of your Mom. Click on below and review Apple’s path forward…

How to request access to a deceased family member’s Apple Account - Apple Support

Take care and post back if we can assist further.

~Katana-San~