It is somewhat valid because we see differences in moral beliefs and cultures, which we see in real life. This is, by definition, what moral relativism is:
Most often it [moral relativism] is associated with an empirical thesis that there are deep and widespread moral disagreements and a metaethical thesis that the truth or justification of moral judgments is not absolute, but relative to the moral standard of some person or group of persons. -Stanford Encyclopedia of Philosophy
Science has somewhat proven moral relativism. A study shows that we are predisposed to certain moral beliefs thanks to our genes the moment we are born, so our own genes play a part in people having different senses of right and wrong. That being said, there is an issue with this statement - just because your morality is subjective, what evidence you use to back it up isn't.
For example, imagine that in the year 1944, there is a nazi soldier who really thinks killing Jews is the right thing. Imagine he feels like he's doing the right thing (much as you would if you gave a sandwich to a homeless person). Then that nazi soldier could be a bad person for you, but you couldn't state he is actually bad because he's not conscious about doing anything wrong. Only by changing his vision of morality he could think he had acted bad.
Technically you could say that, but you have to look at why he thought he was morally justified to do that and whether his justifications disagree with empirical evidence or are contradictory. Many Nazis tried to use claims that Jews were controlling the media through secret Zionist groups. This moral argument goes against what we know about reality, especially during World War II Germany. Same thing with pseudoscientific 'scientific racism' that Hitler tried to use as evidence. So you can still, as a culture, view a person a wrong or incorrect because while the Nazis are free to have their own moral beliefs, the logic utilized to back up these beliefs don't hold up to inquiry.
Allegation: Relativism entails that we have no way to criticize Hitler.
Response: First of all, Hitler’s actions were partially based on false beliefs, rather than values (‘scientific’ racism, moral absolutism, the likelihood of world domination). Second, the problem with Hitler was not that his values were false, but that they were pernicious. Relativism does not entail that we should tolerate murderous tyranny. When someone threatens us or our way of life, we are strongly motivated to protect ourselves. -Philosopher Jesse Prinz in Morality: Nature or Culture?
Also, as some have mentioned, moral relativism is used by many philosophers to talk about entire cultural groups, not individuals. This is called descriptive moral relativism:
Descriptive Moral Relativism (DMR). As a matter of empirical fact, there are deep and widespread moral disagreements across different societies, and these disagreements are much more significant than whatever agreements there may be.
With this, you can argue that we, as a culture, can judge the Nazis, especially individual Nazis in our culture breaking our collective sense of right and wrong. Finally, a moral relativist can still disagree with amorality, moral hypocrisy (their actions or methods in harming a Jewish person in someway contradicts some part of their moral code they claim to follow) and moral absolutism (the belief that your culture's sense of morality should be the right morality for all of humanity and any deviations should be removed), so a Nazi doing these actions because they reject morality as a whole or believe everyone in the world should fully conform to their sense of right & wrong, then even a moral relativist can argue that the Nazi is a 'bad' guy and/or objectively wrong with how they defend their moral code.
