I highly recommend you follow @BobTheFisherman's advice, otherwise you may find out that you may need to perform a clean install some time later when it is less convenient.
This is even more important when an M-series Mac is involved since M-series Macs have a new concept of "Ownership" that will cause you problems. Even an Intel T2 based Mac is likely to give you problems. I have seen many T2 Macs where the T2 security chip gets confused and may require authentication to access the system, but will not show all admin users. I would never trust a T2 Mac or even an M-series Mac without following @BobTheFisherman's instructions. It has nothing to do with you & your mother, or trust between the two of you. It is all about how the new security features of these recent Macs tend to work (or not work correctly as the case may be).
If you ignore @BobTheFisherman's advice, also keep in mind that any apps your mother downloaded & installed from the App Store will be tied to your mother's AppleID so that will cause problems for you. And what will you do when the Mac asks you to authenticate to Activate the Mac for some reason? You may find that your macOS user name is not an option, perhaps it may only show your mother's macOS user account name for authenticating. That would be a bit inconvenient as it would prevent you from accessing the computer. I hope you won't be using the computer for anything critical & time sensitive.
