Super Eyepatch Wolf actually did a really interesting analysis about how Garfield entered the horror genera https://www.youtube.com/watch?v=O2C5R3FOWdE. I click on the video randomly out of curiosity, but I got really sucked in.
Remember, Jon is already talking to a cat who he assumes can't understand him & knows can't talk back. He might as well be talking into the abyss. Only we can read Garfield's inner monologue. Jon's actions are sometimes presupposed by Garfield's whims. This premise is already the basis of some horror or otherwise unsetting fiction.
If Garfield is there or not, if we focus on Jon as the main character of the strip, we might have to do some introspection, whether it's about expecting to have a conversation with cat as if he were your son, that our lives are as boring as his, etc. These are scary thoughts! Garfield's presence serves as a humorous distraction and allows us to forget these thoughts and laugh at Jon, even if briefly. In the same way, Freddy Krueger delivers funny one liners to break up the dread of realizing we're in some sort of living nightmare like people of Elm Street...