People are more popular than people, and horses can't camel than mules. what's the meaning

This sentence means that there is no comparison between people in all aspects, because the differences between people are very large and it is difficult to compare. Horses can't compete with mules because mules can carry far more things than horses. This sentence also implies that people should not blindly compare themselves, because there are great differences between different people or things, and should not compare their own shortcomings with the advantages of others.