if your mom has such perfect credit, and is making more than the minimum due on the car payment, why is it she cant help you out with $200? doesnt she have a credit card you can use and pay it off?
i would say dont be scared of credit cards, and dont be an idiot. i got my 1st credit card when i was like 19, and a couple more over the next few years. taking advantage of 0% offers (not balance transfers, but like when i bought my TV, 0% for 36 months = free money), at 23 i was able to score a 5.000% rate on my mortgage.
i got mine same reason as you, little hard time, and just wanted to have it incase i needed...well, by the time i got the card, i had gotten my next check and was ok. i still held onto it, and reaped the rewards points, and it was nice to have when emergencies came up.
and i know you think you're doing good by being a cosigner, and creditors do have to take that into consideration, but theres going to be a point you dont want to be attached to mommy any more, and itll be nice to apply for credit knowing this is YOUR history, not someone elses you got grouped into.