Cole also doesn't have a surname. So he sees absolutely nothing wrong with Papyrus not having one either! After all, plenty of people don't have surnames. He doesn't. Solas doesn't as well!

However, Cole doesn't really grasp that the 'janitor' part isn't part of his name. So he nods slightly at Papyrus before talking. "It is a name. And it's nice to meet you, Janitor Papyrus."

He can hear Varric's voice in his head at the conversation. Be nice, introduce yourself to people, kid. Everybody likes polite people.