I think I can see where the idea came from that Richard's mother was Margaret Stonor, but the record involved implies that she might also be named Matilda/Maud. See:
Rev. H.L. Elliot (1898) "Fitz Lewes, of West Horndon, and the brasses at Ingrave" Transactions of the Essex Archaeological Society vol.4 New Series.
https://archive.org/details/transactionsess04socigoog/page/n63
He writes "Of the second wife of Sir Richard nothing is known, unless she were one of the ladies mentioned in the Will of Elizabeth Latimer, dated 28th September, 1480. Lady Latimer was the aunt, by marriage, of Sir Henry Fitz Lewes, and amongst other bequests she leaves small sums to Dame Maud Lewes, and Dame Margaret Lewes. The latter may have been the mother of Sir Richard, and Dame Maud this second wife. This however is merely conjectural."
Concerning the connection to the Stonors he says (on the previous page) concerning a transaction of Richard that: "The purchase of merchandise is probably a fiction, and the transaction, as I suppose, recorded the borrowing by Richard Fitz Lewes of £140 from George Stile, on the security of William Stonore, who was probably a near relation of his mother."