When Mormon reviews the wickedness of the people who were living shortly before the birth of Christ, he points out that "at the very time" the Lord prospers the people with riches and other blessings, "then is the time that they do harden their hearts, and do forget the Lord their God." (Helaman 12:2.) Thus, after observing that the people do not keep the commandments of the Lord, Mormon laments: "O how great is the nothingness of the children of men; yea, even they are less than the dust of the earth. For behold, the dust of the earth moveth hither and thither, to the dividing asunder, at the command of our great and everlasting God." (Helaman 12:7-8.)
This statement should not be interpreted that Mormon believes men are worth even less than the dust of the earth. Rather Mormon is saying that men sometimes appear to be less wise than the dust of the earth. When the Lord tells the dust of the earth to move, it obeys, whereas when the Lord tells men to do something, they frequently do not obey.