We expect Kino to sell the pearl and get money, but he ends up throwing it away. Is this an example of irony?

It is indeed. Irony in this instance can be defined as a state of affairs or an event that seems deliberately contrary to what one expects. And this definitely applies in the case of Kino and his family. When Kino finds a valuable pearl, it seems that he's got it made. He and his family will no longer have to eke out a miserable existence, living hand-to-mouth from day to day. With the money they can expect to earn from selling the pearl, they should be able to lead a comfortable life for the rest of their days.
Unfortunately, things don't work out like that. Kino soon realizes that the pearl's more trouble than it's worth. A greedy doctor and his ruthless gang of thugs are determined to get their hands on his valuable treasure. At first, they try to cheat Kino out of the pearl by offering him a derisory amount of money. When that approach doesn't work, they resort to violence, and Kino and his family are forced to run for their lives.
Given that his life has been going downhill ever since he discovered the pearl, we can understand why Kino decides to throw it back into the ocean.