Mormons do not use the cross as a religious symbol, yet it is used as a grave marker in this episode.
While the cross was never a focus in the Mormon (LDS) Church, not using the cross was a gradual thing and, at the time period of this show, it certainly could have been used on graves. For a good discussion of this subject, including when Protestants didn't use the cross search for a Deseret News article, "Mormons and the cross", published in 2009.