It's kind of a big deal, in my opinion, "big public appearances" are all and anything red carpets: Amfar Galas, Cannes, Mostra, Oscars, premières, fashion shows, there are literally tons of events of this kind per year... Being sponsored is kinda tacky but I understand brands need the exposure and celebs need the income streams.
Then you have official states or governments events: state funerals, inaugurations, coronations, G7 or G20 summits, multi-laterals, an so-on, and honestly it's is tacky as f**k to negotiate a sponsorship for these events.
And this guy is a celebrity precisely because he comes from royal family, so he is very much accustomed to those "state" events, he really should have known better. He can do all the Hollywood premieres in Dior, no issue with that (even if it would look greedy ), but that the life he freely chose.
Funny thing is I abhor monarchies, but in some cases formality precedes sponsored deals.