Is it actually? As far as I’m aware, it doesn’t really make any statements that anything is moral or immoral, nor is it a framework that could be used to determine such things by itself, more so a statement on the validity of such things. Or in other word, is it really a moral thesis, or is it a thesis about moral thesis?
You could argue that moral relativism is a metaethical thesis and so is not straight away self-defeating. Even so, moral relativists often go on to claim that we shouldn’t judge the moral acts of other cultures based on what we take to be universal moral standards.
This sounds like Goedels theorem. How could a philosophy be consistent and have an opinion about every moral topic?