I think mine says "please don't take me too seriously because I'm really just a silly little lol cat." It's a non-verbal INFP apology for any gaffes I'm inevitably going to make. Plus, I have a pretty twisted sense of humor (I love black comedy), and if I said dark things with a scarier icon it could be taken very much the wrong way.
Of course, this can pose a problem when I'm actually being srs.:steam:
Sometimes I'll be defending something I feel passionately about, being ever-so-cynical and critical, then I'll glance at my icon and imagine the little kitten stomping his tiny paws in an adorable fit of temper. That doesn't help my credibility in arguments much, I suppose.