Could also be that Ratbert is only feigning attention to Dilbert's statement in order to butter Dilbert up a bit and get himself some of that sandwich... Being Ratbert, though... Who knows whether he'd just assumed Dilbert was saying something funny or whether he actually thought Dilbert's complaint was humorous lol.
His request for a bite of Dilbert's sandwhich is further proof that Ratbert simple and carefree. He probably doesn't even care that his fur changes colours. He's probably been testing hair products at the lab.
I'm not sure whether Ratbert is clueless or random this time... Does he think the sandwich is the assignment, or is he just being random after he stops laughing?