Not all politeness is inconsistent with communicating truth. I agree that "Does this dress make me look fat" has a true answer and a polite answer. It's worth investing some attention into figuring out which answer to give. Often, people use questions like that as a trap, as mean-spirited or petty social and emotional manipulation. Crocker's Rule is best understood as a promise that the speaker is aware of this dynamic and explicitly denies engaging in it.

That doesn't license being rude. If you are really trying to help someone else come to a better understanding of the world, being polite helps them avoid cognitive biases that would prevent them from thinking logically about your assertions. In short, Crocker's Rule does not mean "I don't mind if you are intentionally rude to me." It means "I am aware that your assertions might be unintentionally rude, and I will be guided by your intention to inform rather than interpreting you as intentionally rude.