I've always thought that was kind of hilarious -- I mean, Occam's razor. What's more likely, that some random kids with a few beers in them were acting the maggot with spray paint and drew a penis on an uninhabited farmhouse (which, in fairness, is what TW says), OR that someone read and disliked TSP, somehow got hold of the names of the new tenants, put two and two together and decided to express their opinion of their (former) homelessness on the walls?
SW emotes: Someone must have read the book; they know we’ve been homeless. That’s why it says “scum” and now it won’t wash off. I should never have let it be published. This is what happens when you let people see into your world.
That's a classic instance of combined SW glumwashing and Main Character Syndrome. It's not pleasant that a house you're about to move into got graffitied, no. But it's a bit of a reach to bring it back to a negative review of your bestseller!
I think that's what it's about. Like the imaginary disapproval of 'tramps' from people on the SWCP and the ill-fitting boots in LL, and of course TW's 'illness' throughout all the books, the interpretation of the graffitti here is reminding us that the Walkers are loveable victims, down on their luck and hapless and encountering prejudice, despite the fact that SW's bestseller is earning nicely, she's sought after for literary events and interviews, and their new landlord, a huge fan, is contemplating signing over this house to them because of his regard for them.
But SW works very hard to make sure we never see them as comfortably-off, sought-after, and in a position to pick and choose.