I've been struggling to make sense of it. The white queen is the Roman goddess Fotruna and the only depictions of her with a bared breast I've found are modern art. Usually a bared breast is either meant to represent nurturing/charity or an indication of sexual violence.

Sure, it could mean be a symbol of her being nurturing, but in the context of this set, I wonder if it is a reference to Nazi depictions of ancient mythology such as the Night of the Amazons where they used a warped image of the mythology to display women naked amidst their "master race" propaganda. Fortuna was also popular in Germany at the time given Carmina Burana was written in the 1930s.

Ultimately I came to the same conclusion about decadence as you, but I'm not sure if there is anything more to the symbolism than that.