I think her casting was excellent. She was a WASPy type. Michael Coreleone was very different from the family. College...Dartmouth. Marine. War Hero. He wanted something different from what he grew up in initially. So it makes perfect sense he would find his gf/wife outside of his family’s world. It made the story much richer to have that pull and tension and finally conflict. If he had had a stereotypical American-Sicilian wife it wouldn’t have been as rich. Boring in fact. And after the attempt on his life and the killing of his new bride, it makes sense when he comes back to America that he reaches again to something outside of that world. Normalcy. But of course, he gets pulled back in...famous line in 3 of course.
The movie made a ton of money so, what can I say?