perhaps Frà Pandolf chanced to say, "Her mantle laps Over my lady’s wrist too much," or "Paint Must never hope to reproduce the faint Half-flush that dies along her throat": such stuff Was courtesy, she thought, and cause enough For calling up that spot of joy.
The Duke imagines some of the ways that Frà Pandolf might have caused the Duchess to get that "spot of joy" in her face.
He might have told her that her "mantle" (her shawl) covered her wrist too much, which is the Renaissance equivalent of saying, "man, that skirt’s way too long – maybe you should hike it up a little."
Or he might have complimented her on the becoming way that she flushes, telling her that "paint / Must never hope to reproduce" (17-18) the beautiful effect of her skin and coloring.
The Duke thinks the Duchess would have thought that comments like this, the normal flirtatious "courtesy" (20) that noblemen would pay to noblewomen, were "cause enough" (20) to blush.
Strangely, the Duke seems to believe that blushing in response to someone like Frà Pandolf was a decision, not an involuntary physical reaction. Notice that the Duke also seems to infuse his comments with a judgmental tone.
She had A heart – how shall I say? – too soon made glad. Too easily impressed: she liked whate’er She looked on, and her looks went everywhere.
The Duke describes the Duchess as "too soon made glad" (22) and "too easily impressed" (23). This is his main problem with her: too many things make her happy.
Another way of looking at it is that she’s not serious enough. She doesn’t save her "spot of joy" for him alone. She’s not the discriminating snob that he wants her to be.
She likes everything she sees, and she sees everything.