Will I be able to earn credit card cashback if I top up my GrabPay wallet using my cashback credit card?

Amex true cash back card works in this way.

Short answer, yes.

Unless! That card excludes Grabpay top-up. Otherwise, to the bank, it is just another transaction and it will reward you the cashback.

But if your card has a minimum spend per month, do ensure that you hit it or else your cashback will be reduced.