Considering all the injunctions to be of good cheer (e.g. 3 Nephi 1:13), one might expect the Book of Mormon to have a positive use of the word merry. In the Book of Mormon, the term merry is always used in a negative way.
The term merry is used five times in the Book of Mormon. It is used of unbelieving Lamanites twice (Mosiah 20:1; Alma 55:14), the wicked twice (2 Nephi 28:7-8), and of Laman and Lemuel once (1 Nephi 18:9). So in the Book of Mormon unbelievers are merry.
The Book of Mormon also associates the term with a variety of specific behaviors. It is associated with singing and dancing twice (1 Nephi 18:9; Mosiah 20:1), with drinking (especially wine) and drunkenness three times (2 Nephi 28:7-8; Alma 55:14), as well as rudeness (1 Nephi 18:9).
So the Book of Mormon uses the term merry for a certain type of light-heartedness, associated with the wicked, who drink and get drunk. It is the enjoyment of the wicked in their wickedness so that "they did forget by what power they had been brought thither" (1 Nephi 18:9).
This is a contrast with Joseph Smith's day, when, according to Webster's dictionary of his day, to be merry was to be "Pleasant; agreeable; delightful" or "jovial."
So while the Book of Mormon might encourage us to be of good cheer, it does not want us to make merry.