The ending does have some aspects of moral closure to it. Take for example how Huck's attitude towards Jim has changed from the beginning of the novel. At the beginning, he viewed Jim as property, and as an ignorant human with no brains or rights at all. He even gets upset at the audacity that Jim shows in wanting to buy his wife and children back once they reach slavery. However, by the end of the book, Huck views Jim as an equal--he even states, "he was white on the inside," which goes to show that he felt Jim was worth thinking of as an equal. He also considers Jim to be a good friend, and goes to great lengths to help him to escape. So, the moral closure there is that Twain helps a young boy who was raised in a slave-owning state, and who was indocrinated by slave owners, to see that slavery is not moral and that slaves are human beings who deserve rights also.
Another example of moral closure comes when we find out that Miss Watson had freed Jim before she passed away because she felt so bad for separating his family. This shows that not all the slave owners were evil people, and some did make moral choices in the end. They recognized that slaves had families, and were hurt when torn apart. So, there is a bit of moral closure there too.
As for other areas where moral closure could've occurred, Twain leaves it pretty open. For example, the Duke and King get away with all of their shenanigans, so, criminals who did great harm to people are let free. Also, many of the evils that Huck had encountered throughout the story are not resolved; Twain simply shows a lot of the negative aspects of society without proposing any solutions or hope. In the area of Jim and Huck, and a few people's attitudes towards slavery however, he does provide closure.
I hope that those thoughts helped; good luck!