This can depend. Do you live with your mom full time? If so, yes - you probably need to be on her policy. If you don't, you will probably have a harder time finding a car insurance policy that will insure a non-resident driver. This is simply how car insurance policies are. Companies will require the vehicle be insured in the zip code it is parked at the majority of the time. If it's not your mom's house, you might have issues. Here
is a guide for finding cheap car insurance.