More on guarnello
"GUARNELLO (M/F). Both a kind of linen or cotton textile, and the feminine garment constructed from such a fabric.... The guarnello, rascia or saia is the standard form of dress for angels. It is worn by children as a simple, washable garment, and possibly also by pregnant women. There are also examples of guarnelli listed under items of male clothing." - Dress in Renaissance Italy, 1400-1500, by Jacqueline Herald, Humanities Press: NJ, 1981, quoted here

The coarse overdress for working women definition is from The World of Renaissance Florence by Francesco Adorno, Translated by W. Darwell, Giunti, 1999, ISBN 8809013492.
page 80

This site says:
A loose, sheer overdress (worn over a regular dress), was used at home and after birth - hence very few depictions of such garbs exist. Some has argued that it is not too unlike the sheer chemise like garbs Botticelli's "Three graces" wears in his "Primavera", but it can also be seen in his portrait of "Smeralda Brandini" in London.

That may also be following Herald, which is listed as one of the sources.

The fabric is also translated as "fustian" which would seem to be the coarser variety.

And here's the kicker: An article in the Globe and Mail (quoted with the crucial next line here) says
Mona Lisa had recently given birth to a baby, a team of Canadian and French scientists announced Tuesday, so her mysterious smile may have expressed the weary joy of a mother with a newborn. Using infrared technology that allowed them to see beneath a layer of varnish, the researchers found that Leonardo da Vinci's model had a gauzy layer over her dress they say was typically worn by pregnant women of the time, or mothers who had recently given birth. The filmy robe was called a guarnello. 
-