I don't think this came about by randomly deciding to take a character out of a strip. The creators of G-G recognized that Jon is a depressing character who has these one way discussions with Garfield. Garfield's inner monologue (that Jon is not aware of) provides all the humor, mostly at Jon's expense. Take the humor out of the comic, and you're left with the depression. It goes straight from a comedy to a tragedy.

This is what's interesting about G-G. The tragedy was always there. We kinda knew the tragedy was always there, but we'd rather laugh at Jon with Garfield than commiserate with Jon.

Taking superheroes out of a random movie would lead to silliness, yes, but nothing poignant.