No. The source the article gives for this is an extremely un-scholarly one, a (self-published?) book on fashion history that focuses on individuals and stories associated with them. The author is mixing the well-known painting of her as a nursing Madonna with contemporary criticisms of her and other fashionable women showing skin: Georges Chastellain, a chronicler of her period, wrote that she wore "open robes in the front where one can see her breasts", for instance. Unfortunately, interpreting criticisms like these is more difficult than just going, "guess she wore gowns completely open in the front!" Necklines that were low enough to show the top of the curve of the bust could be sometimes considered, by conservative commentators in the late Middle Ages, shocking and indecent and an example of "showing the breasts", as could gowns that were tight enough to outline the body. It's also possible that there was a fashion for leaving the gown loosely laced in the upper body, allowing a peek at the chemise and skin beneath it. And finally, we shouldn't forget that her position as the first (at the time, only) official mistress of the king of France also made her a target for condemnation that could, in theory, exaggerate the truth in order to make a point against the immorality of the king openly consorting with a woman who was not his wife.

There are no images of women of her period doing this apart from the portrait in the diptych, and no extant gowns that show evidence of having been worn that way (but in all fairness, extant clothing from this period is extremely scarce).