I think you could claim several moods, but I would use disenchanting as one descriptor. This is supposed to be told in a fable or fairy tale. Thus, there should be something happy, we expect that, but all we experience time and again is dissappointment in what Napoleon further does to the animals with each new chapter. We are also disgusted to watch the society of animals just continue to take the propaganda without revolting or rebelling. Particularly Americans want to see the animals claim the rights that they know they deserve after animals take control, so once again, we arrive at disenchanted. This is good though because when we read dystopian literature we begin to look at our own societies and see where we have had leaders have too much sway.
The story actually goes through several moods. If one knows the tunes, the song "Beasts of England" is funny because one can imagine the animals singing it to "La Cuchuracha" and "O My Darling Clementine." There is hope for the animals at the beginning when they overthrow Jones. The animals have a clear plan to establish their own utopia. However, soon you realize that there is a problem here; the animals cannot read, so they do not know what the commandments of animalism are except for what they are told. You feel a sense of foreboding about Napoleon from the very beginning, when he sneaks off with the milk from the cow soon after the revolt. You feel sad for Boxer who literally works himself to death. You feel anger at Napoleon and the lesser pigs for exploiting their "comrades." One might even feel a certain sense of disgust with Benjamin, who only says that donkeys live a long time and will not do anything to help the situation when he is the animal most likely to help. The story ends when the revolution goes full-circle and the animals are no better off than they were; if anything, they are slightly worse off because the pigs are even more intrusive than Jones. You feel sad for the animals, especially the ones who remember the ideals of the revolution.