Oliver Cowdery initially wrote mattereth here in the printer’s manuscript, but then somewhat later (perhaps while proofing against the original manuscript) he changed mattereth to matters with slightly heavier ink flow (he crossed out the final th of the -eth ending and overwrote the e with an s).
Elsewhere in the Book of Mormon, there are 11 occurrences of “it mattereth not”, including one that is very similar to Mosiah 13:9, namely “and whither I go it mattereth not” in Mormon 8:4. There is also one example of “this mattereth not” (in Alma 40:8). On the other hand, there is one other example in the text of “it matters not”:
The higher frequency of “it mattereth not” suggests that Oliver Cowdery initially wrote mattereth because in this biblically sounding expression (for which not follows the main verb matter), Oliver expected the -(e)th ending more than the standard English -(e)s ending, which is what we would expect if the phrase had been “it does not matter”. In addition, the use of the archaic whither rather than the modern where adds to the biblical style of this sentence. Moreover, all the other examples of “it mattereth not” have remained unchanged throughout the textual history, which argues that here in Mosiah 13:9 there would have been no motivation for Oliver to have changed mattereth to matters except that 𝓞 itself read matters. Finally, there is one other example where Oliver accidentally supplied the -eth ending to this expression:
In this instance, Oliver accidentally replaced matter with mattereth as he copied from 𝓞 into 𝓟 (𝓞 is extant here); thus we have clear evidence that he expected the phraseology “mattereth not”.
Summary: Retain in Mosiah 13:9 the corrected reading in 𝓟: “it matters not whither I go”; the initial reading, mattereth, is expected, so the correction to matters most probably represents the reading of the original manuscript, no longer extant here.