It is very unpolite to peep into a lady's jewelry cassette and then expecting that the lady MUST wear something out of her cassette.
The contents of someone's jewel cassette are in principle a private matter. We have no idea how the situation is at the palais princier. Does Charlène have jewels at all? And if she has jewels (the Ocean Spray, the Repossi collier) does she own it, or were it loans and does she actually like these jewels?
I think the Princess knows the invitation and how it works at the Scandinavian courts. And if she did not know, the courtiers at the palais princier should have known, let alone her own husband. I think the Princess had no any intention to be rude or disrespectful so we can only guess about her choice to appear pretty low key. Still it is her own choice to which she is perfectly entitled to.