I think Alan is correct, that was the sound of Linda's particular voice up high. Though no note was ever lost I found myself wondering once or twice if she was singing out of her range or if in fact these were just notes within her range that she hadn't cultivated, that were out of her comfort zone. This was my impression both times I saw the show.
While there are countless sopranos who could have sung Mabel better than her, I don't think there were many rock stars who could have sung the role at all. Linda had the sort of success with "Pirates" that Brando had with the film of "Guys and Dolls." Others may have been better for the roles overall, but they challenged themselves successfully. (Brando was a better singer than Linda was an actress, I have to say.)