Symbolism, Imagery, Allegory
The cows that invade Lena's evaluation are a statement of protest by the Invalids. They're to intended to represent that, in this futuristic, government-controlled society, "We're all a bunch of herd animals" (4.63).
Although the protest does have some an effect on Lena, do you think that it is really that effective? It only temporarily delays the evaluations; it doesn't stop them forever. And in fact, we never see the Invalids stage any sort of action that has a real deterrent effect on the oppressive government regime.
Are the Invalids just biding their time until they take more dramatic action against the government, or would the Incompetents be a better term for this group of rebels?