p1. Miserable as Garfield may make him (see panel 3!), if this is what Jon does with his spare time, keeping Garfield around may also serve the function of keeping Jon's life interesting. Not that this is much different than how I spend my spare time. Note also that Garfield himself spends even more time "zoning out", but less for the reason that he is dull, than a deep satisfaction with himself.
p2. Comics Technique 101: The relative size of the onomatopoeia sound effect is most frequently depicted in correlation to the volume of the sound it depicts. This is not the only possibility, however, and equal power may be derived from a subjective depiction of the sound. In the above Garfield strip, the sound of a plastic dish full of soft canned food landing on a man's skull which is padded with poofy hair, probably does not make such a loud "clop" as to warrant filling the entire panel with the noise. But the effect of portraying the sound as Jon hears it, and the added ability of the lettering obliterating our view of the action, maintains the mystery of the source of the noise for the third panel reveal.
p3. Bless again the decades of winnowing down Garfield comics shorthand. Three lines under Jon's eyes are all the "reaction" needed to convey his exhaustion and frustrated acceptance of Garfield's antics. We do not need to see the perpetrator to know that Garfield threw his dish, nor are we confused by the single-word horseshoe pitching joke in a thought bubble extending from off-frame. In Garfield, every simple element is weighted and clear, a model of comics refinement.