I think the only one who pulled it of was John Qualen, a master of accents & dialects. But giving the movie makers the benefit of the doubt, these were American ancestors of Mexicans living in the 20th Century. I could see some dilution of their accents.
I thought Spencer Tracy's language was appropriate (but his dark make-up was not.) Danny and his friends are a version of the knights of the Round Table. Tracy's archaic and stilted manner of speaking helps to place them in those mythical times. Also, according to Steinbeck, Paisanos are a mixture of Spanish, Indian, Mexican and assorted caucasian bloods who speak English and Spanish with a Paisano accent, which could be any sort of accent because Paisanos are a people made up by Steinbeck.
I'm not sure if you're referring to her Austrian accent which makes her more exotic in this role or her on-screen presence, but either way I can't disagree with you. I haven't seen all of her movies but in every one I've seen she lights up the screen as much as she does here.