Napoleon may not be great with a crowd, but he's "especially successful with the sheep" (5.8). He teaches them simple cheers, has them chant at strategic times during public meetings, and even hands out pompoms.
Okay. No pompoms. But we kind of wish there were pompoms.
Napoleon's strategy is successful: "It was noticed that they [the sheep] were especially liable to break into 'Four legs good, two legs bad' at crucial moments in Snowball's speeches" (5.8). Later, Napoleon uses the sheep to chant and drown out the four pigs who protest when he announces that he'll eliminate public meetings.
Because the sheep are so, well, sheep-like, they're super easy to manipulate. At the end of the novel, the pigs start walking on two legs—so, Squealer teaches the sheep a new chant: "Four legs good, two legs better" (10.13).
Obviously, the sheep are part of the problem. They're dumb followers who go along with the crowd. But there's another problem: chanting makes everything too easy for the leaders. It's simple; it's repeatable; it can be manipulated. In other words, Orwell is saying, beware simplistic slogans. Whatever side you're on: if you can chant it at a rally, it's probably not a well-thought out political philosophy.
The Sheep and Stalin's Russia
The sheep are part of the massive propaganda machine that Stalin set up as he came to power in Russia, and they're also the people who were swayed by that same propaganda. Instead of thinking for themselves, they just repeat slogans over and over. By the end of the novel, we can't tell the propaganda apart from the people—er, sheep—who spread it.
The Sheep Timeline