He didn't need to act in most of his movies because that's what his audience wanted - John Wayne. They didn't want him wandering too far away from the character they paid to see.
He could act plenty. For a start, he acted Jeff Bridges (an actor I have a great fondness for) off the stage as Rooster Cogburn. As he told tales of his escapades to the impressionable young girl in the mountains, yep, you believed them. He told them with the lucid memory of great acting.
And when he raged at his companions in The Searchers: "what do you want me to do, spell it out for you?" when asked to describe the remains of a girl killed by Indians, that steaming malevolence is brute cold.
Great Actor - John Wayne