Many people use "chrome dome" to describe bald people. It's just a nickname used to distinguish people. Especially when you're surrounded by 40 + people day in and day out. It can't be easy to remember 40 + people's names so Sawyer just uses nicknames instead. That way, if someone were to come up to him and say "You seen Hurley" Sawyer can say "Oh, Jethro. Yeah, he's in the jungle." Or whatever. You get the idea.