The Lord responds that some of what Moroni fears will be part of the process: “Fools mock.” It will happen. It has happened. When the final accounting comes, however, “they shall mourn.” These things are true, and truth will be attested at the last day.
The next lesson is one that easily expands far beyond the context of writing the Book of Mormon, or even its reception. God teaches us a lesson about our humanity. We have weaknesses, and God “will show unto [us our] weakness.” Those weaknesses can be explicit. We might hide them. We might not even realize we have them.
Nevertheless, God “give[s] unto men weakness that they may be humble.” We must understand that we rely upon God, and turn to him in faith. Of course, we also have strengths, and all humankind understands that we can do for ourselves in those things where we are strong. It is in our weakness that we most often realize that we should turn to God.
The miracle that comes after that exercise of faith is that God will then “make weak things become strong unto [us].” Sometimes that does mean that someone weak in writing or speaking might become strong in writing and speaking. Sometimes it means that even if that particular thing never becomes significantly better, we do. We become stronger by founding ourselves in faith, particularly faith in Jesus Christ.