He only cares about what people think of him but he never follows through with anything. He is OBSESSED with looks and is extremely superficial but he is so stupid that he only capped the front four teeth, never considering the new shiny white front teeth only make the other ones look even dirtier and more rotten.
Hence, the metaphor. Kodys teeth are a window into his soul. He only cares about surface level things and never digs deeper to discover the true cause of the rotten, brown fangs and horrible breath.
It’s kind of like the wives. All the relationships seemed fine until a new shiny wife entered the picture. After Robyn, the rot started to show and smell.