This article explains why Garfield sucks so badly: Jim Davis planned it that way.
Davis meticulously plotted Garfield's success. And part of his calculation was to make the strip so inoffensive that it's hard to hate it even for being anodyne. [...] he consciously developed a stable of recurring, repetitive jokes for the cat. He hates Mondays. He loves lasagna. He sure is fat. The model for Garfield was Charles Schulz's Peanuts, but not the funny Peanuts of that strip's early years. Rather, Davis wanted to mimic the sunny, humorless monotony of Peanuts' twilight years.
And here I thought he had just run out of ideas.