There's a kind of social teasing that's common in humanity. Some sources of it are verboten, socially -- like race and even, in some cases, sexual orientation. Other sources can be considered semi-acceptable targets, like religion or the size of your ears. It's the things that make us different or stand out from each other that become lazy crutches for ridicule or ribbing.
Which isn't to say Mel Gibson isn't an anti-semite. But there are circumstances where people say anti-semitical things out of laziness more than any real racial hatred.