Even in an unrighteous world, not all people are unrighteous. They may be in the minority, but their faith is still beneficial to them. In Mormon’s case, his faith was strong enough that he
“was visited of the Lord, and tasted and knew of the goodness of Jesus.” That was powerful faith, and undoubtedly part of his call as a disciple (apostle).
As with other disciples who had testimonies of Jesus Christ, Mormon wanted to preach, but his “mouth was shut, and [he] was forbidden” to preach.
The key to the Nephite wickedness was that “they had wilfully rebelled against their God.”