Does the reward for charity decrease if I give money to my mother for her to distribute it to those who are deserving but whom I do not know, especially if my mother has already given the charity on my behalf, with me intending to repay her later?

If you do not know the needy, it is better to give charity to your mother for her to distribute. And secret charity is better than open charity unless the openness of it is a reason for emulation. The intention must be sincere for Allah Almighty, seeking His countenance. There is no harm for your mother to give out charity with the intention that its reward be for you, provided you repay her the amount, and thus the reward will be for both of you.