According to his Marine Corps records he was twice rated as a sharpshooter.
If it is unreasonable that a Marine Corps-trained sharpshooter, on a clear and sunny day, got off two out of three good (from his perspective) shots - well then, nothing is reasonable.
When Oswald lived in Minsk he belonged to a gun club there and members also reportedly said he was a poor shot.
Let me put it yet another way. Marine Corp Sniper Instructor Carlos Hathcock, who shouldn't need any introduction, examined the shots and tried to duplicate them at Quantico. When constraining the course of fire to be as exact as possible, not one of the marine corp snipers could make those shots.