I have no idea if he was a liar or not.
Just what did he claim to have done that he didn’t do?
Do some Internet searches to look at his falsifications of his military service and deecorations. Note how he eentered service as a commissioned officer due to his law degree, and how he was an intelligence officer who flew 12 missons as a rear gunner and observer. Then look at what he claimed instead. Also take note of the falsified commendation letter. Bottomline: Kerry copycatted McCarthy.
While you are at it, you may want to take a look at Chon, McCarthy’s assistant. Pay close attention to Cohn’s career back to his support of McCarthy.