So I know that at this moment of time, public approval rate is just for role-play reasons and I thought that perhaps this can have potential in-game values as well. With government type and policies as modifiers, here is my thought: at around 60%, the material production will decrease from its usual rate. At 35% there will be infrastructure damage from the "riots" that erupt from the streets. Less than 10% and your population begin to whittle away. All three of these aspect do stack up on each other and continue to grow more significant as the percentage nears zero.