According to Daniel Ludlow, the reader should note that the main story in the book of Mosiah is told in the third person rather than in the first person as was the custom in the earlier books of the Book of Mormon. The reason for this is that someone else is now telling the story, and that 'someone else' is Mormon. With the beginning of the book of Mosiah we start our study of Mormon's abridgment of various books that had been written on the large plates of Nephi. (3 Nephi 5:8-12.) The book of Mosiah and the five books that follow--Alma, Helaman, 3 Nephi, 4 Nephi, and Mormon--were all abridged or condensed by Mormon from the large plates of Nephi, and these abridged versions were written by Mormon on the plates that bear his name, the plates of Mormon. [Daniel H. Ludlow, A Companion to Your Study of the Book of Mormon, p. 173]
One might ponder the difficulty Joseph Smith would have faced had he been inventing the text of the Book of Mormon. He would have had to make a literary switch from dictating a third person account (Mormon's abridgement of the large plates was apparently translated first) to a first person account. Historical accounts reveal that he apparently dictated word for word to Oliver Cowdery and other scribes without hardly any corrections (a fact which becomes apparent when viewing the relatively "clean" surviving portions of the Original Manuscript). [Alan C. Miner, Personal Notes]