This is, of course, a matter of opinion. There is no way to objectively determine when something is or is not moral. My own view is that dumping is generally immoral.
Dumping is the act of selling one’s products abroad at a very low price. This price is lower than what the good sells for at home and is often lower than the cost of producing the good. The point of doing this is to drive foreign competitors out of business. If you can dump your products in a foreign country, the low prices may destroy any domestic makers of competing products. Once you have destroyed them, you can raise your prices and make more profit.
It is possible to argue that there is nothing wrong with dumping. After all, companies within the United States are free to sell their goods at prices that are below the cost of production. This can simply be seen as a form of capitalistic, free market competition. Any company that is capable of absorbing losses in the short term, in this view, deserves to win its competition with other companies.
However, I would argue that dumping is wrong even if it is legal. A company that does this is typically not competing in good faith. It is using its size and its ability to absorb losses to destroy other companies. It is not outcompeting them by having a better product or lower production costs. Instead, it is simply throwing its weight around. Therefore, I would argue that firms should not sell their products abroad at anything other than a fair market price.