Literary: There is a contrast made here that is somewhat obscured by the separation of verses. To understand them properly, we need to restitch them thus:
…but thou hast not made us mighty in writing; for thou hast made all this people that they could speak much, because of the Holy Ghost which thou hast given them; And thou hast made us that we could write but little, because of the awkwardness of our hands.
Moroni contrasts writing with speaking, and suggests that they might be mighty in speaking, but awkward in writing. For Moroni, the difference is the Holy Ghost. When speaking he can feel the electric communication that accompanies teaching when the Holy Ghost is present and enhances our abilities. In writing, the audience is distant, and we cannot feel that powerful two-way communication that is above our abilities. That is power in speaking. Awkwardness in writing comes when the spiritual feedback is not present.
[thou hast not made us mighty in writing like unto the brother of Jared]: Moroni is surely referencing the revelation he has read of the brother of Jared’s experience with the Lord and subsequent vision of the whole plan for the world. It is most probable that most of what Moroni references is in the sealed portion of the plates.