I think it's important to view the Pipe Strip in philosophical terms... We've touched briefly on the notion of existentialism; that theme is very prevalent in this strip. Garfield is, in fact, a modern existential anti-hero... but if Garfield embodies the bewilderment in a meaningless life, what is Jon? What are the telltale signs that inform Jon's philosophical standpoint? His approach, what style of thinking he represents?
Jon is depicted as being grounded in the material world... a world of things; he is surrounded by objects, and he touches these objects, he interacts with them. The newspaper, the end table, the chair... his clothes, all these physical things make up Jon's world. In some sense, even his cat Garfield is an object to him, a thing...
The first ideology that comes to mind when thinking of objects in the tangible world... is pragmatism... Is Jon Arbuckle a pragmatist? His beliefs stem from a useful, coherent view of his environment... a sort of cause-and-effect understanding of his world helps him.
A: Deduce that his pipe is missing... and B: Catches his cat, Garfield, using the pipe.
This kind of empirical and logical thinking lends credence to the idea that Jon is, indeed, a pragmatist... Although, it is hard to entirely ignore the rest of the Garfield comic canon.
While Garfield is consistently anarchic, and embraces the chaos and absurdity of life... Jon Arbuckle exhibits an erratic, unpredictable mix of philosophical behaviors. At times, he is borderline; delusional, an idealist, an almost slap-happy version of Don Quixote. Other moments, he is rigid, nearly to the point of being obsessive... somewhat like a structuralist, and certainly has streaks of sarcasm and negativity that might classify him as a skeptic.