The animals are not well educated. Does that make them foolish? They suspect that things are going awry and that the commandments are being corrupted, but they lack the education to be able to prove it.
Their suspicions suggest an intelligence that they simply fail to translate into education and action.
This is true, to a certain extent. The animals readily "buy in" when Napoleon and Snowball come up with the plan to overthrow the farmer and start their own rule. When Napoleon progresses with changing the rules they originally established, most of the animals continue to go along with it. As an example, in chapter 3 the animals learn that the pigs have been eating the milk and not sharing with the other animals. It says, "At this some of the other animals murmured, but it was no use....Squealer was sent to make the necessary explanations to the others." Squealer serves to smooth things over whenever a pig makes a decision the other animals might not like. In this instance, he tells the others that Jones would return if the pigs didn't get enough nutrition; the other animals go along with the plan, then, because they don't want Jones back. Later on, when the animals want to disagree with the pigs' decisions again, Napoleon presents his "puppies" as a form of intimidation. Now the animals are too afraid to speak against the pigs, for fear the dogs will attack them.
In some ways, then, it can be said that the animals caused their own problems. If they had all spoken up sooner, or refused to believe Squealer's version of the truth, they might have managed to overthrow Napoleon before he got too powerful. But his promises were intended to hide the real truths and coerce the other animals, so they can't be entirely at fault.
In response to post #5, always is a strong word. What about American Revolution? There are many more examples of very successful and desirable revolutions.
It adds to Orwell's point about the futility of revolutions; that the revolters always end up as bad as the people they overthrew.
Majority of the animals on the animal farms are about as foolish as majority of people in any country. Ordinary people do not understand how to govern their countries. Frequently they also do not have access to reliable information to take an informed decision on right way to manage.
Thus people do get swayed by ideas presented to them by skilled orators. Also when people do disagree with the existing system they are not always able to change it immediately. For example, it took nearly a hundred years and a civil war for American people to officially remove slavery in USA.
Once you are under an oppressive rule, it is not all that easy to overthrow it. Therefore, I prefer to consider animals on the Animal Farm as animals with average intelligence rather than foolish.