Almost certainly, craft. that is why they have serene highnesses. Our own Queen Mary, consort of George V, was only a serene highness on marriage. In her case, it was because her mother was a royal princess, of our royal family, but her father was only a German duke. Since their marriage was one between two people of unequal standing (a 'morganatic marriage') their daughter merited only 'serene' as her title. (How nice that such snobbery existed at such a high level!). It's a fair assumption that Monaco's Grimaldi family felt it improper to label daughters, or consorts. Royal, when their father or husband was a mere Prince, not a descendant of a king, and thought that 'Serene' was as much as they could do by way of a title.