1980s Garfield is a cat, and most of the humour is based in his behaviour as a cat, and his relationship with his human Jon, and a few other animals and people.

2020s Garfield doesn’t really do anything cat-like, and in 99% of the newer comics, could just be replaced by “a wisecracking guy who talks to the reader and delivers the punchline”. You could swap him for a weird old man or child who lives with Jon and it would basically be the same.