If you live outside of your mother's household and borrow her car and are involved in an accident, typically her insurance will extend to you as a driver. If instead you live in the same household but are not listed as a driver on the policy for some reason then the insurance company may not cover you for operating her vehicle.

In order to place a claim for the scratches to the bumper your mother would need to have more than just liability coverages on the car. Liability covers damages you, as a driver, caused to another person's car or property, not damages to your own vehicle. Your mother would need physical damage coverages of collision and comprehensive to cover damages to her car. In this case it sounds as if she would make a claim through her collision coverage.

Now if you did have insurance of your own, your mother's would still be primary. Auto insurance usually follows the car first and driver second, so if you had insurance it would have been secondary or excess if damages exceeded your mother's limits.