Sans isn't paying attention to what Rarity is doing, exactly, because it's clearly work, and who cares about that. Instead, Sans is once again casually in their house with a level of ease that makes it feel like he lives there now. At the moment, he is in thier kitchen doing some lazy ass baking.

So why not distract her from being constructive?

"Hey," he calls to her from the kitchen, "what kind of shit should I put in this pie?"

Allies or no, he still finds being intentionally crass in Rarity's direction sort of funny.