Undeserved kindness and Grace are the same thing. JWs simply don't understand it because they don't understand the concept of freedom from sin or forgiveness.
It means, you can never Earn it, you can never do anything to Deserve it, and it matters not how much you do. It is freely given because God WANTS to and loves you. Truly accepting Christ means your sins are being overlooked (as long as you don't sin against the Spirit or just keep doing the same rotten thing over & over).