While the "joke" is self-explanatory (and really, who doesn't enjoy shitting on Heathcliff?), I'm guessing that the larger point behind this strip is that Heathcliff is some kind of battletank, capable of completely ignoring acorns and bird dung as he takes his destructive, broken-footed stroll through the neighborhood. He doesn't even look annoyed with the squirrel's antics, most likely because he's about to cut down the telephone pole and sodomize him with it.