She is no Sovereign Princess but just the wife of.
She has no institutional obligations except just being the wife of.
The Bal de la Rose is no institutional obligation. It is a fundraising event for the foundation of which Princess Caroline is the organizator. Therefore she is the Dame d'Honneur and her family is attending. It is just a party, no ceremony of state...
In essence it IS a small and meaningless affair. It is a bal. A dance event. And rich foreigners have to pay fees to attend, the revenues go to a charity. No more, no less. Every day there is a fundraising event somewhere in London. The Queen never attends. Boohoo to her too, then?
The big affairs are the Fête Nationale, Sainte-Devote, Christmas, the Laureus Awards, the Grand Prix de Monaco, the Diamond League, international congresses and symposia in Monte-Carlo, not an evening of dance and drinking champagne in a club. Come on...