A dystopian novel as one in which an author creates a world that is a total nightmare to live in. That is the opposite of a perfect world.
In this novel, Napoleon creates a world that is not exactly the worst possible, but pretty close for the animals.The world that Napoleon creates is one where the animals (pigs excepted) have no power. They have to do what the pigs say and they are liable to be killed if they do not. The pigs use the other animals for their own gain. That is no better than it was when the humans ran the farm. In fact, it is even worse because now the animals have their hope totally dashed. They thought animalism would improve their lives but it isn't at all .
So this is a dystopian novel because Orwell depicts a world that is really unpleasant to live in.