This is a good question. The Pearl is an excellent book with many different morals. So, there will be differences of opinion. In my view, the chief moral of the book is that greed and ambition are destructive forces. Just a look at what the desire for wealth does. There is an escalation of tragedy in the book on account of greed.
First, it starts with a innocent dream of having something better for himself and family. For example, Kino says:
"In the pearl he saw Coyotito sitting at a little desk in a school, just as Kino had once seen it through an open door. And Coyotito was dressed in a jacket, and he had on a white collar and a broad silken tie. Moreover, Coyotito was writing on a big piece of paper."
Second, this desire leads to people wanting to steal the pearl or cheat Kino to get the pearl at a cheaper price. So, we can say that the pearl not only influences Kino but others as well. The desire for wealth even makes Kino hit Juana, his wife, who wants to get rid of the pearl. Kino even winds up killing two people in the story. And finally in the mess of trying to get to the capital, Coyotito, the little boy, is killed. In the end, nothing good happen all because of greed.
What is most ironic is that at the end of the story, the pearl is thrown back into the ocean.