It was a formal, black tie White House supper, official White House china and all (the table is also set formally). She was not overdressed. Since she was the hostess of the party, she sets the dress, and everyone else is judged accordingly.
ITA
I actually think the dress is beautiful. (for once!)
I grew up with these rules too and the first one was to try not to “over-do” or “out-do”, or make your guests uncomfortable. Black tie is not the same as being an actor on the red carpet. Most of this weeks dresses worn by the actresses would not be appropriate for the White House event.
They have consultants and a separate book of rules for the White House. I wonder if it got burned with some other famous books? LOL