As someone who also remembers Garfield being funny as a kid, I don't read the OP as saying it was never funny, but that being funny was a secondary concern to Davis after being marketable. I just looked up a random Garfield strip from 1988 [1] and I have to say, it's kind of a funny visual gag. Nothing groundbreaking but for a daily strip in a newspaper, it's not bad, and I'm an adult now, as a kid I probably would have found it hilarious. Combine decent humor like that with recognizable and cute characters appearing in the newspaper daily and it's not surprising that the strip became so popular.
[1] http://images.ucomics.com/comics/ga/1988/ga880101.gif