To your first point, would you argue art is not the field of human interest with beauty as its ideal? Would you argue art strives for ugliness? Art holds ugliness and "badness" as its ideal? I find that to be a very untenable position to hold.
"To produce the beautiful, art must imitate nature. Yet not every imitation of nature, merely because it is an imitation, is therefore beautiful, as realism pretends. The reality imitated by art must also be beautiful, or art must add to it the idea that will give it beauty." Br. Louis de Poissy
"Beauty and goodness in a thing are identical fundamentally; for they are based upon the same thing, namely, the form; and consequently goodness is praised as beauty." Summa Theologica P1 Q5
Guernica imitates reality as it is a depiction of a war crime. However, just because it is a depiction of reality, it does not mean that it is beautiful. Further, it does not depict the beautiful nor give it an idea that makes it beautiful. As you say, the grotesqueness, horror, and chaos depicted was intentional. You yourself say it is ugly, and I agree. Following Aquinas's thoughts above, if beauty and goodness is fundamentally identical, then it follows that the antithesis of beauty and goodness is also identical. Therefore, since this work of art is the antithesis of beauty (ugliness, as you yourself say), it is also bad art, not to mention that the bad is not praised as the beautiful. I hope this answers your question.
"To produce the beautiful, art must imitate nature. Yet not every imitation of nature, merely because it is an imitation, is therefore beautiful, as realism pretends. The reality imitated by art must also be beautiful, or art must add to it the idea that will give it beauty." Br. Louis de Poissy
"Beauty and goodness in a thing are identical fundamentally; for they are based upon the same thing, namely, the form; and consequently goodness is praised as beauty." Summa Theologica P1 Q5
Guernica imitates reality as it is a depiction of a war crime. However, just because it is a depiction of reality, it does not mean that it is beautiful. Further, it does not depict the beautiful nor give it an idea that makes it beautiful. As you say, the grotesqueness, horror, and chaos depicted was intentional. You yourself say it is ugly, and I agree. Following Aquinas's thoughts above, if beauty and goodness is fundamentally identical, then it follows that the antithesis of beauty and goodness is also identical. Therefore, since this work of art is the antithesis of beauty (ugliness, as you yourself say), it is also bad art, not to mention that the bad is not praised as the beautiful. I hope this answers your question.