I think you're overthinking it. I think overwhelming majority of people don't want that crap over their streets. It would be an easy 80+% issue for a politician to pick up so most places have laws that say don't have that ugly crap everywhere. Hence you see the value of neighborhoods with a lot of graffiti and considerably lower than those that don't

Is graffiti causing those neighborhood's value to drop or are businesses and individuals residing in cheaper neighborhoods less equipped to cover the ongoing maintenance costs of removing the ever-recurring graffiti?