I write and sing (hopefully) humourous songs. I use the humour to hide a not too brilliant voice. When people are laughing, they aren't thinking "he's got a voice like a bullfrog". I find that people laugh the most at songs where I am making fun of myself. Maybe its because I feel more comfortable making a fool of myself rather than having a go at someone else. It is a good way of keeping your feet on the ground - looking at what you do that is daft, stupid, pathetic, etc.