There are two possible explanations for this strip. First, Heathcliff is an illusion, and Mr. Nutmeg wrapped the garbage cans for some unknown reason, then decided to rebuke his imaginary cat for something that'd be impossible for him to do, let alone it being a behavior he's prone two. Second, this Heathcliff is a Heathcliff from a mirror dimension, replacing the normal, evil cat with a nice, tame, bearded house cat that does family chores and puts gift wrap on everything.

Either way, I fail to see how this is funny.