You speak no Latin more than I, belike; However, you're my man, you've seen the world —The beauty and the wonder and the power, The shapes of things, their colours, lights and shades, Changes, surprises,—and God made it all! —For what? Do you feel thankful, ay or no, For this fair town's face, yonder river's line, The mountain round it and the sky above, Much more the figures of man, woman, child, These are the frame to? What's it all about? To be passed over, despised? or dwelt upon, Wondered at? oh, this last of course!—you say. But why not do as well as say,—paint these Just as they are, careless what comes of it? God's works—paint any one, and count it crime To let a truth slip. Don't object, "His works Are here already; nature is complete: Suppose you reproduce her—(which you can't) There's no advantage! you must beat her, then."
Lippo aligns himself even more with the guardsman by pointing out that neither of them speak Latin. That's important, because during the fifteenth century (when the real Lippo lived), Latin was the language of the super-educated and Church class. This downplays Lippo's status and puts him more on the same social level as the guard. It makes the guardsman Lippo's "man" (282).
And what's more, they both appreciate the world as it is, Lippo suggests. And since that's the way God made it, it's not evil (although this is more implied by the monk).
Should everything—the beauty of the town, the river, the surrounding mountains and sky, as well as the people that inhabit it—just be ignored or hated?
Always the artist, Lippo gives us another helping of metaphor. The world is the "frame" (290) for the "figures" (289) of men, women, and children that dwell within it. His artist's eye zeroes right in on how he sees the world: everything is an object to be painted and considered in an artistic manner.
You're probably wondering at the enjambment that's going on in this passage. The bro's getting all worked up over his topic, so the poetic lines here reflect that spillover. (Check out "Form and Meter" for more on how this poem is put together.)
Now Lippo takes on a hypothetical voice that questions creating art at all. Since God's work is already present in the world, why paint it? It's already complete, right? And since you can't reproduce nature (here personified as a woman), an artist has to "beat" her and make his works cause the viewer to contemplate spiritual things.