There are two important changes in this verse from the one in Isaiah, and both come from adding the word “not.” Isaiah said that the “mean man boweth down,” and that the “great man humbleth himself.” Apparently, Joseph understood this verse without reference to what came before, and the negated the phrases. Thus, in the Book of Mormon, the mean man and the great man are to be condemned because they do not bow down or humble themselves. That sentiment would be correct if it were Jehovah before whom they bowed and were humbled.
In Isaiah, that isn’t the case. The subject is the foreign idols, and Isaiah is condemning those of Israel who have welcomed the idols. They bow down before, and humble themselves before, the foreign idols. Therefore, they should not be forgiven.
When Isaiah notes both the mean man and the great man, he is using two extremes to suggest all those in between. The mean man is the poor or lowly person. The great man is the one with wealth and power. The combination of the two is a literary device that is intended to mean everyone. Isaiah is condemning the entire nation for accepting these foreign influences. Along with the wealth, they have accepted the foreign gods. That is not to be forgiven.