The original use of art here in Mosiah 2:21 seems to be intended. The original Book of Mormon text sometimes used biblically styled inflectional endings in nonstandard ways. One prominent example of this is the original use of the -(e)th ending with first person singular verbs, in particular the occurrence of “I saith” in the historical present tense (see the discussion under 1 Nephi 11:3). Of course, the correct verb form in Early Modern English for art here in Mosiah 2:21 would be is. The 1906 LDS large-print edition introduced this emendation, and it has continued in the LDS text. The RLDS text has retained the original nonstandard use of art in this verse.
Here the biblically styled art is conjoined with a preceding use of hath (“who hath created you from the beginning”). A similar example of this kind of combination in the biblical style is found later in the text:
In both Mosiah 2:21 and Alma 5:37, we can be fairly confident that the art is not a mistake for are. Modern English speakers expect are only with plural subjects, so it is doubtful that the use of art in these two passages is an error for are. Note, by the way, that in Alma 5:37 the 1841 edition reads are instead of art, but this is probably a simple typo rather than an attempt at editing the text. The following 1849 LDS edition restored the original art. (As with Mosiah 2:21, the 1906 LDS edition made the correct grammatical emendation to is in Alma 5:37.) Of course, this 1841 example of are does show that there are cases where art and are have been accidentally mixed up. For a discussion of such mix-ups in the manuscripts, see Mosiah 2:24 below.
Summary: Restore the original use of art in Mosiah 2:21 and Alma 5:37; in both cases art is conjoined with a preceding hath and represents instances of the biblically styled language of the original Book of Mormon text.