A particular royal bride(whom I will not name) has had a very unconventional past; yet has become the Crown Princess of a particular Nordic country.
I presume you're talking about Princess Mette-Marit, not Princess Mary, correct?
As the others have said, I think that unless Princess Angela is being ignored in ways that the wives of other Liechtenstein princes are not, I doubt this is a matter of prejudice, just mostly a matter of not concentrating on royals that hardly anyone has ever heard of. It's sort of a case of being damned if they do and damned if they don't, because I think that if she was given a lot of publicity on account of being the only black member of a European royal family, people would be accusing the media of not being able to see past her skin colour and hence of acting on the basis of discrimination.