Facts and Events
One theory is that she was a Wampanoag princess, but this has been soundly disproven in TAG 1938.
Another theory is that she was a sister or relative of Rev. Marmaduke Matthews of Wales. This is supported by the following from Source:The history of Malden, Massachusetts, 1633-1785.
p. 131: At the age of 33 he [Rev. Marmaduke Matthews] arrived, with a company of west-country people [from Wales], in a ship of Barnstaple, at Boston, September 21, 1638. His wife, Katherine, came with him, or followed soon after...
p. 146: List of 36 women on petition dated 28 Oct 1651 in support of Rev. Matthews, including Marg't Welding. [It does not indicate if signed or marked but appears to have been written by scribe since some women are named with titles, not names.]
p. 158: He [Rev. Marmaduke Matthews] returned to England in 1655, and several of the Malden church went with him. Of these returning pilgrims I can recognize the widow Margaret Wheldon, who left a law-suit over the estate of her deceased husband, Gabriel... [Citing Mass. Colony Records, iv. (v1), 151.]
p. 158: But little is known of the family of Mr. Matthews. His wife, Katherine, who came with him from England, was a witness of the will of Gabriel Wheldon in 1653, as was Michaiah Matthews, who may have been his son.
p. 158, the following footnote about Gabriel Whelden: Gabriel Wheldon, or Welding, who appears to have been a personal friend of Mr. Matthews, was with that minister at Yarmouth, and took the oath of fidelity with him [in 1657 in Yarmouth]. He came here [Malden] with Mr. Matthews, and in his will calls himself "of the Towne and church of Mauldon". With his youngest son, John, he sold to William Crofts, of Lynn, four parcels of land in Arnold, county Nottingham. Essex Deeds, i. 24. This forbids the conclusion that he was a fellow countryman of Mr. Matthews; but from the apparently close connection of the parties, I am inclined to believe that his wife, Margaret, was from Wales, and perhaps owned a relationship with the pastor.