Why don't we use the plural of lazy in this sentence?
Because, just like in English, adjectives modifying nouns (being in front of nouns) stay singular in Hungarian.
But if it had said, "Pigs are lazy," then would it be lustak?
Yes, exactly! "A disznók lusták."
Because then it is the predicate ("Állítmány") of the sentence, not just a common adjective ("Jelző" in a sentence).