So, is this because Trebuchet are not particularly lethal, or because they were not used much?
Trebuchets were used for a variety of purposes in siege warfare, from (uniquely) breaching walls with large boulders to (less uniquely) setting things on fire to (probably) suppressing massed formations of archers with smaller shot. But smaller shot would not leave a characteristic trebuchet injury, it would leave a generic blunt-force trauma injury.
I learned that from Monty Python and the Holy Grail.