Looks like she is wearing an undergown with a sleeveless robe over it. People did not, or could not clean their fine fabrics like velvet or brocade like we can today with chemical (dry) cleaning. Until the late 19th century most ladies wore an undergarment - a shift - made of something that could be washed. It served as their underwear by day and their nightgown by night. Mona Lisa probably has a linen shift under the black "robe" she is wearing. The black robe or dress is probably made from velvet or another fine fabric and would be removed, aired and stored at the end of the day but there would be no way to clean it except for "spot cleaning" of spills. Over the black robe is a sleeveless embroidered and ornamented garment. This would be a very fine garment and used over many different long sleeved robes of a woman of wealth. I have heard that the Mona Lisa (or La Gionconda) is pregnant? Prior to the 20th century, women's garments were all "pregnacy friendly" - the lacings could be taken in and out or a loose garment could be belted or unbelted to accomodate a ladies condition.
I've worked for Colonial Williamsburg for 35 years. Williamsburg is centuries after Mona Lisa but until the "Pill" the story of ever changing waistlines in ladies fashions was always a consideration.