Jon's surprise in the second panel is, for me, what elevates this slightly from a normal "pet food is gross" joke. It's also an observation about the unpleasant information revealed when reading in full any food label. Since he obviously purchased the Winged Things cat food, brought it home, and got all ready to feed Garfield before even glancing at the contents: "I bought this?"
More startling than a cat's blind desire to eat any kind of bird, regardless of how good it might taste, and the disregard for human squeamishness on the part of the pet food manufacturers, is the third panel revelation that the key ingredient is an artificial additive. Perhaps "zing" isn't the only flavor-experience Winged Things has going for it, but Garfield seems unimpressed until he hears about the sparrow-flavoring. For our purposes, this means the food company is blithely killing exotic animals and violating a minor cultural taboo against eating raptors, pretty much for no reason. Garfield can get his fill of real sparrows in the backyard any time. For Garfield, even eating his dinner today becomes less about nourishment than making Jon squirm, and unnecessary destruction of animals more beautiful than himself. Good show.
As always: would any other strip make a running gag out of reading a canned food label?