Ether was righteous, and Moroni gives some details to let us know that he was. The reason is not simply to indicate that he was righteous, but to create the contrast between a prophet who was sent to give the people of Jared one last chance, which they reject. The contrast dramatically places Ether on the side of the God, whom he has seen, and the people who reject him and therefore implicitly follow the Adversary. They are ripe for destruction, and Ether is the witness to it, just as a righteous Mormon was witness to the Nephite destruction. Moroni does not make that comparison explicit, but he would have understood it.
One of the interesting aspects of the declaration of Ether’s righteousness is that he has a vision of the future of the land. One of the things he sees is the New Jerusalem. That name is certainly one that the Nephite translator added to Ether’s text. The Jaredites left the Old World at a time that they may not have had any record of the Old Jerusalem, and therefore would not understand the concept of a New Jerusalem. Nevertheless, seeing the future of the land, Ether would see the establishment of the seat of God in the New World, even if he did not understand it by the name we have learned to call it.