Was the Garfield comic strip the first place this exact phrase was used?
Google Ngram Viewer indicates that that phrase didn’t show up in published works until 1984. Given that Garfield was first published in 1978, and really hit its stride – with both syndication levels and merchandising – in the early '80s, I suppose it’s possible.
We were using it as kids at least 10 years before Garfield debuted.
It was a variant of BFD which when asked by “sensitive” people meant “big fuzzy duck” although “big fat deal” was also sometimes used as an explanation.