Here the 1837 edition replaced “wax stronger” with “wax strong”. Occasionally, there have been mix-ups in the text of comparative and base forms of adjectives; for a list of examples, see the discussion regarding the word nearer in 1 Nephi 2:5. Here in Alma 21:3, both the LDS and RLDS texts have retained the 1837 reading with strong rather than the original stronger.
The use of the comparative with the verb wax does occur elsewhere in the Book of Mormon text, but only in the repetitive expression “wax stronger and stronger” (Helaman 3:35 and Helaman 11:37). Other examples of repetitive comparatives are found in the King James Bible: “wax louder and louder” (Exodus 19:19), “wax stronger and stronger” (2 Samuel 3:1), “wax weaker and weaker” (2 Samuel 3:1), “wax greater and greater” (1 Chronicles 11:9 and Esther 9:4), and “wax worse and worse” (2 Timothy 3:13).
Elsewhere in the Book of Mormon text, the verb wax almost always takes the base form of the adjective. There are 20 occurrences with strong as the adjective and 13 with other adjectives: old (5 times), great (4 times), and bold, hard, pale, and proud (once each). However, there is one other case where we have a comparative adjectival form, but this is with the periphrastic more:
Thus there is one other example of a nonrepetitive comparative adjectival form occurring with wax.
The text here in Alma 21:3 is saying that the Amlicites and Amulonites were causing the Lamanites, who were already wicked, to become even more wicked. Thus the use of the comparative stronger is perfectly appropriate in this passage and will be restored in the critical text.
Summary: Restore in Alma 21:3 the comparative stronger since this is the original reading and it makes perfectly good sense in this passage.