This phrase has more than one level of meaning. Inequality in mortality is rarely blamed on sin and transgression, but the sins of pride, greed, deceit, stealing, and dishonesty all breed such inequality. If all were righteous and living the law of consecration, there would be no inequality at all. But it is not given that one man should possess that which is above another, wherefore the world lieth in sin (DC 49:20). If inequality is because of sin, then the converse should also be true—that equality is a function of righteousness. The law of equality, under the law of consecration, is a necessary and fundamental component of celestial law, That you may be equal in the bonds of heavenly things, yea, and earthly things also, for the obtaining of heavenly things; For if you will that I give unto you a place in the celestial world, you must prepare yourselves by doing the things which I have commanded you and required of you (DC 78:5-7).
The second level of meaning to Mormon’s comment is that there is inequality in the rewards given to men. This inequality also comes because of sin and transgression. He is contrasting the death and punishments of the wicked, consigned to a state of endless wo (v. 11), to the death and rewards of the righteous, raised to dwell…in a state of never-ending happiness (v. 12). In many respects this inequality is greater than that of mortality. In other words, the difference between the blessed state of the righteous and the cursed state of the wicked after death is much greater than the disparity between the rich and the poor during mortality.