This seems to come up here every now and then. While cleaning buildings in your town or a neighbor's town will fix it, it does not seem that having criminals roaming around or having spray-painted buildings drops it.
Some people have suggested that the righteousness rating falls randomly. I think that it drops when the game decides you need a new quest - like when it tells you to collect money from shops or restaurants, or decides that Cletus really, really needs to grow more Silvertongue. It's basically telling you, "Hey, go visit your neighbors!" I just raised mine back up from 1 star to 5 by performing 8 actions: sprayed then cleaned a building in 1 town, tapped a snake log, cleaned 2 buildings in another town, tapped a shop, then returned to my town & cleaned 2 buildings.
I keep meaning to take a more scientific approach when this happens but then I forget. I know for sure that there were 0 dirty buildings when I logged in, and Legs, Louie, and Snake are in the slamma! Even Mayor Quimby is being good right now, writing a book on ethics. ;)