It strikes me as unlikely that graffiti would have been so ubiquitous if only 10% of the population were capable of reading it. Should we assume a basic level of literacy was widespread. Perhaps people couldn't decipher the flowery language of Cicero, but they could make out the sexual puns or political slogans scrawled on walls?
I could swear I wrote a lengthy reply to this identical question some years ago, but I can't find it now. On a side note, it is virtually impossible to search one's own reddit posts now. Not even the two external search tools work anymore. A bit baffling in 2021, but here we are. Fortunately, I was able to find better answers than my long-lost attempt from these other fine folks:
https://www.reddit.com/r/AskHistorians/comments/apjcw7/what_was_literacy_like_in_ancient_times/