This Just In: Marilyn Didn't Suck!
The answer, I guess, is that this was six years earlier, on the more functional end of her death spiral, and she’s amazing. The song is done in long takes, with fairly complicated choreography (though you couldn’t really call it dancing; its more like a form of timed prop-brandishing.) And if IMDB is correct, she even did the bulk of her own singing! (Only the high notes were dubbed by that musical-dubbing workhorse, Marni Nixon.) Pure entertainment. It now goes next to Rita Hayworth's "Put the Blame on Mame" scene in Gilda in my list of favorite movie song solos by women.
By the way, another thing that surprised me is that, hard upon that first performance, Jane Russell (Marilyn's allegedly co-va-voomy co-star in the film) does her own reprise of “Diamonds” as well, and there’s a reason it’s not famous. It’s the musical equivalent of a blunt object—which is pretty much the way Jane Russell is in all her films. Howard Hawks is famous for directing tough women, so maybe it did something for him personally. But I’ll take Marilyn, even though it’s hard to argue the movie even offers a choice. Despite their co-billing, Jane Russell is very much Ethel Mertz to Marilyn’s Lucy. Marilyn’s even got better comic timing!
Anyway, I’m going to sleep happy now, and I’ve learned a lesson about not prejudging.
NOTE: Corrected as of 5 June; Rita Hayworth was in Gilda, not Roberta as I originally reported in my sleep-dazed state. Thanks, Esther!