A lot of us hated the Pies as a kid...I know my Mum and Dad where Melbourne supporters...me a Bombers fan, and it was just natural, because they were the natural enemy..they were big, strong and considered as dirty...and had incidents to back that up. They also had R.C. (Catholic) connections, way back...and in Melbourne at the time, there was a bit of a divide between religions, not now of course.
However..these derogatory terms are more from the younger generation..they probably think it's funny.
Anyway, the club I came to hate was the Lions, because the had all those built in advantages, plus they played in an energy draining climate and for a few years they never lost at home.
Wheres the Pies now are a respectable outfit who play great footy. I wouldn't mock them, but again those people who do that are really on the same level as the people they are mocking.