I have not been able to find anything about her.
From Wikimedia; spots, linear flaws from paint cracks, and veiling reflection suppressed with Photoshop with mixture of suppressing individual spots and flaws throughout image and blurring parts of the background with Photoshop by gogm.
Mary's dress is a lovely and flattering robe à la Française and an attention-getting set of hair jewels that stand out against her dark hair in this 1763 Gainsborough portrait.
Her bracelets can be seen here, her engageantes can be seen here, her hair jewelry and lace neck ornament here, and her lace neck band and hair jewelry can be seen here.
Keywords: 1763, Gainsborough, Mary Little, Little family, Carr family, British, straight coiffure, hair jewelry, ribbon headdress, neck band, lace, bows, neckline ornament, square neckline, modesty piece, elbow length sleeves, lace sleeve ornaments, engageantes, sacque back, revers, stomacher, panniers, earrings, bracelets, robe à la française