Foolishness in Animal Farm takes it root in the lower class animals, who are essentially duped into a life of hardship because of their lack of intellect. They fail to recognize the horrible nature of their oppression, the greed of the pigs, or the worsening of their lives. Foolishness also takes its root in the desire for petty but meaningless things, in vanity and frivolity. Mollie, who loves ribbons and sugar, just can't to adjust to a life of hardship after the Rebellion because of her foolish desire for the finer things in life.
Although the pigs manipulate the working class animals with many different tools, it is ultimately not those tools, but rather the inherent foolishness of the animals themselves, that leads to their easy oppression.
Although the pigs are more intelligent than the rest of the animals, the humans ultimately make them into fools. In Animal Farm, we see that foolishness is a universal quality.