(Taking notes from the comments here...)
No, questions asking "Is this polite?" should not be on-topic
These questions are primarily opinion-based. That is, it invites answers which are justified simply by saying "Yes, that seems polite to me" -- but then someone else can just as easily say "No, I would be offended by that", which is equally valid as an answer.
The help center even explicitly states that questions are off topic which:
- ask us to adjudicate “right” and “wrong” in a situation or whether something is rude/racist/sexist/[insert other discriminatory terms]. If you are having a dispute with your spouse or coworker, we are not going to settle the dispute for you or give you points to argue. Focus your question on resolving your issue, rather than whether there is an issue or not.
If it can be answered with some level of objectivity and reputable sources, then it is more of a question of understanding established norms, and can be edited to make that clear (and should include enough context to be reasonably scoped).
The question A server in a restaurant got offended when we addressed him as "Uncle" is a good illustration of the differences. Right now it includes both types of questions:
Is this correct or we were wrong in thinking so? How should one address servicemen/servicewomen in the restaurant in a proper way?
The first sentence is asking users to judge the situation: was this polite? You can see how the answers give varying opinions whether the OP was impolite or the waiter was overreacting. Simply hearing a "yes" or "no" from random internet users is not very useful in developing any skills, which is what we're all about!
But, the second sentence is asking about cultural norms: what is the proper way to address waiters in Germany? The answers addressing this part are useful and can be applied in future situations. They are also more objective because they can be backed up by personal experience or references, which is another hallmark of a good answer.