Their deception is likely to be successful in the short term. Papyrus has never been great at reading Frisk's expressions, and that's unlikely to change in the course of a dream. What he remembers afterwards, however, might disappoint them both.

"Well, I wasn't suggesting you build a snow-Sans! Yours would almost certainly be better than his. And that might hurt his feelings!" He intends to build another snow-Papyrus, of course. His face is best possible subject for artistic creations.

Perhaps this one should be in a car, with cool shades, instead of flexing to impress people. Driving is even more cool than big biceps.