Hotels used to do this all the time after they cleaned your room. The picture implies they're 100s of miles from anyone, but someone cleaned his campsite and left him a mint. (I think)
There is a lot of ambiguity here and I think that was what Larson found funny.
These two hard-bitten cowfolk are out in the middle of nowhere, clearly no-one around but each other. It is obvious that the sleeping cowboy placed the mint...but why?
The contrast between the rough environment and the tender act if mint placing is already kind of funny. Your mind then starts to try to complete the story. Was this a romantic gesture? Or maybe it was something more sinister. A reference to past misdeeds thought forgotten?
Larson loved ambiance and sometimes used it in place of a punchline.