This is an intelligent question. Slim is, without a doubt, the most respected man in the whole book. This would make us believe that he does not experience alienation, but he does. There are two reasons why.
First, because people respect Slim so much, no one really gets to know him. The language which is employed to describe Slim is remarkable. He is godlike according to the men. I will quote a lengthly portion of text to give a sense of what Slim is like:
When he had finished combing his hair he moved into the room, and he moved with a majesty achieved only by royalty and master craftsmen. He was a jerkline skinner, the prince of the ranch, capable of driving ten, sixteen, even twenty mules with a single line to the leaders. He was capable of killing a fly on the wheeler’s butt with a bull whip without touching the mule. There was a gravity in his manner and a quiet so profound that all talk stopped when he spoke. His authority was so great that his word was taken on any subject, be it politics or love. This was Slim, the jerkline skinner. His hatchet face was ageless. He might have been thirty-five or fifty. His ear heard more than was said to him, and his slow speech had overtones not of thought, but of understanding beyond thought. His hands, large and lean, were as delicate in their action as those of a temple dancer.
The irony of Slim's greatness in the eyes of the men is that no one gets close to him. He is set apart in an almost religious way. In this sense, he experiences alienation.
The second reason why Slim experiences alienation is on account of the context. Steinbeck creates a world where there are no friendship, no community, and no relationships. No one cares for each other, and no one takes the time to get to know one another. In a migrant world, there is no sense of permanence. So, why even build community? Because Slim lives in this world, he experiences alienation. In this sense, it is not about Slim; it is more about he world in which he lives.