Sans actually draws back a little in surprise. It takes him a second to accept and understand that Mettaton...really did just say that.

His response, when it comes, is a little quieter despite still being a growl. "Good. Nice." The tip of his tail twitches, just a bit. "You're nice."

Surprisingly so. Is this the Mettaton that Frisk gets to deal with? It's...kind of nice, being friends with him. Even like this. Sans will try to enjoy it while it lasts.