She was not well-coordinated. Old Joe Kennedy could not tolerate losers and it was a huge problem to be the odd girl out when assembled with all the athletic Kennedy's. Who would not be depressed if you could not keep with your brothers and sisters and your father read you the riot act because you were not nimble like the rest of them?
Dr. James W. Watts, who performed the lobotomy on her, is quoted to have said that at the time of the procedure Rosemary was suffering from depression and not mental retardation. In the 1940's, even though a lobotomy was new and really experimental, it was never done for metal retardation but only for mental illness.
The Sins of the Father by Ronald Kessler
What a tortured family.
Old Joe was much worse than most people realize, working with the Mafia, cheating all his friends, chasing his sons' girlfriends, lobotomizing Rosemary while his wife was out of town. It was suggested in one book (Sins of the Father?) that Joe had the operation done because Rosemary was talking too much. That was connected with his fondness for young women. I don't know how much can be asserted about all that. A fraction of the truth about Joe is bad enough.
The family was not tragic. God gave them what they brought upon themselves.