The lines of the CK make me think of the court dresses in the 1750s, which may explain the cleavage. I think it's actually an intentional nod to this style. In my opinion, she pulls it off - most women would look completely ridiculous.
She rarely wears anything I hate. Even if it isn't my favourite, I have to admit she picks interesting pieces. They always make me look twice - really intriguing structures.