I would like to get a refund on my mother's registration fee since she just passed, however, I am not the executrix.  Is this still possible?
Unfortunately, no relevant information is found.
How can I receive the refund of my mother's registration fee?
It is possible to request that refund, however DMV will issue it to the name of the estate, so you would have to work that out in a way for you to receive it then.  To request the refund of your recently passed mother's registration fee, you must surrender the vehicle plates at a DMV office and request a transfer receipt, not a refund receipt.  You need to have the executor or executrix of the estate send a completed and signed Request for Refund [1] MV-215 with "deceased" on the form, a photocopy of the death certificate, and the transfer receipt to the address listed on the Request for Refund form.