Very likely. However, partial lobectomies are done all the time. It was long ago and my memory of that one conversation isn't complete, but I'm fairly certain there was a possibility of saving him with less impairment than you suggest.
Most of the pictures, or x-ray reproductions, or drawings, both official and conspiratorial, that I have seen - and I'm not a Kennedy conspiracy fan - all suggest that the bullet went through JFK's brain in a fatal path. Add to that the energy that the bullet imparted to the brain matter, I doubt if there was a functional brain left there, even if the contents had remained in the skull. Poor b*st*rd.
Oddly enough, you mention "lobotomies". It was JFK's sister, Rosemary, who had received a lobotomy for her mental illness. The whole Kennedy family tale is nothing but a story of abject tragedy. The "Luck of the Irish" for the Kennedy's was all used up by "Old Joe" Kennedy. From Joe Jr. to JFK Jr., the old bootlegger left no "Irish Luck" for the rest of his family.