you know, I have never thought of this, sadly. Of course, depending on the dungeon there might be lots of graffiti - and could add story hooks or other interesting immersion. I will certainly use in my next dungeon.
"The pie is a lie!" *some blood and scratches* "Da orc isnt!"
I imagine the most common graffiti in dungeons would be glyphs like the ones described in the World of Greyhawk folio or the Forgotten Realms Campaign Set. Admittedly, I've never used these much (if ever). I might have to change that
I would expect the development of a set of pictograms very similar to the hobo signs, especially for well known dungeons. Even if you don't like them for dungeons an entrance to the darklands might be another place to use dungeon pictograms.
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.