In his editing of the printer’s manuscript for the 1837 edition, Joseph Smith crossed out the much in this passage. The emended text (“not so concerned about his people”) is more formal sounding. Nonetheless, the 1837 edition never implemented this change. Perhaps it was decided that the original phraseology (“not so much concerned about his people”) sounded perfectly fine. All subsequent editions have maintained the original reading, as will the critical text.
There are no other passages where so much is followed by an adjective or an adjectival participle. We ignore one secondary instance that was introduced into the text in the 1837 edition: namely, the change of “so exceeding more numerous” to “so much more numerous” in Alma 58:2. For discussion of a case where Joseph Smith removed much from before a past participle, see under 1 Nephi 18:15.
Summary: Accept in Mosiah 19:8 the original “not so much concerned about his people”; Joseph Smith’s deletion of the much in 𝓟, motivated perhaps by stylistic considerations, has never been implemented in any edition.