I've seen it disputed that she was retarded at all. I have seen the case made that she was somewhat mentally ill but not of an IQ below 85 before the "Pyscho Surgery". But I can't cite the reference or how legitimate the claim may be.
At the time lobotomies were used mostly for the violently mentally ill who might otherwise hurt themselves and others. Nowadays such behavior is controlled by drugs instead of surgery. It was highly questionable to have a lobotomy performed on someone who wandered, talked to herself, disrobed at inappropriate times or masturbated in less-than-private circumstances. But in that era there were other lobotomies performed that would certainly be viewed as shockingly unethical at an earlier or later time period.
Yet the drugs pretty much do the same thing. Affect their brains, make them dull, listless and easy to control.
I've been trying to remember what my mother said about when I was a kid (we're in Boston). I do recall she hated Joe Kennedy for it. Of course, she had no use even for rich people who kept their kids in boarding school and/or summer camp year round. I think her opinion was that Rosemary was just "boy crazy" and a bit wild.