Fortunately (?), Papyrus is well-versed in looking at eyesockets instead of mouths, when it comes to reading expressions. He has to be, if he's to make any sense of his own reflection. So he sees enough to see that something, somehow, bothered his brother about what he said. Not enough to see which part, or any hint of why.

"Of course I would!" Indignant, like they've had too many conversations about what Papyrus does and doesn't think already. "If this is some new joke routine, I have to tell you, it's not funny! At all! You should stick to puns."

If that doesn't say how unfunny this conversation is, nothing will. Sans' overuse of bad puns has been a thorn in Papyrus' side for years. Better that than this.