If your mother could not observe the fasts because of her medical challenges or poor health, then you should offer Fidyah for her missed fasts. If, however, she just neglected her obligation to fast, then you need not do that.

Having said that, I would not stop you if you wish to do that as a kind of spiritual gift you want to bestow on the person of your own good will and leave it to Allah to accept it.

Almighty Allah knows best.