Worst casting choices in Hollywood history had nothing to do with race and ethnicity. Some actors just plain suck at certain roles. Redford sucked as Gatsby and Cruise sucked as LeStat.
But you gotta admit, John Wayne was brilliant as Genghis Khan.
Well, pretty good.
Uh, adequate.
OK, he sucked.
I agree with your first statement (some actors just plain suck in the role, regardless of how much they look or sound like the character). But for your second example, I actually thought Redford was THE definitive version of Gatsby, and Cruise was THE definitive version of Lestat. Both casting choices worked perfectly.
Gatsby is SUPPOSED to be a good looking, shallow, vapid, snooty rich guy. Redford could just pretty much be himself and pull it off. (DiCaprio was excellently cast as the 2013 Gatsby for the same reason... that's not to say the rest of the movie is good, it isn't) The more controversial casting in 1974 was Mia Farrow as Daisy. People apparently had a hard time buying that in the 70s, but now the movie is so well known its hard to imagine anyone else as Daisy.
Lestat's casting was up there with the announcement of Michael Keaton as Batman. Everyone hated it at the time and couldn't picture the actor in the role (especially since Lestat is supposed to be a 6'4 tough guy with beautiful blond hair in the book, whereas Cruise is a 5'7 mellow guy with dark brown hair). But Keaton shocked everyone with how much he owned the Batman character, and Cruise did the same with Lestat -- he captured the character perfectly on screen -- the smug cocky "brat prince" who was depicted in the original novel as a completely unfeeling sociopathic killer that you hated but still came across as charming and likeable. Anne Rice even hated it but changed her mind after she saw the movie and praised his performance.