I think that you'd be most likely to see graffiti near the entrance of a dungeon, as blustering adventurers try to leave their mark before they go in, or in the outlying areas of humanoid lairs where bored guards or angry adolescents write rude comments.
Deep in a dungeon the adventurers will be too busy -- and, having made it that far, probably too serious-minded -- to be painting on the walls. And the monsters are probably also the more serious* kind.
That said, I'd love to trick my group by having graffiti indicating a fake "portal". You know, a previous adventuring party had one bored member tagging along at the back who decided to have a laugh and write "Beware portal!" on a blank wall.
* In every sense of the word.