Bradbury uses Clarisse to express that humanity as we know it is on its way to being extinguished... just like Clarisse. All those things that made Clarisse alive  (the ability to appreciate smells, sights and sounds, the ability to think and ponder, the desire to learn) were values or qualities that the society either frowned upon or moved away from because they all had something to do with feeling or thinking. As readers we like Clarisse and her affect on Guy, but as the storyline continues we realize that she just vanishes. It's ironic that this happens to her because this is a part of Bradbury's message. If we do not do something with the way that technology is robbing humanity of thinking, feeling, and relating, we too are on our way to permanent extinction. It is a dreary message, but certainly touches readers closely as they consider how lazy behaviors and attitudes affect them.