how-to-answer has a lot of good information on how to correctly answer a question, but doesn't address what NOT to put in an answer. There was an answer recently to a performance question which was specific to one part of the platform (not going into detail in order to not make it searchable), but the answer went on about general performance for that platform. Should there be a point about being on-topic? Asides that are relevant to the question are great, but going on for quite some time about marginally related topics merely clutters the answer.
No, we shouldn't. For the same reasons we don't need an explicit policy about sarcasm, or warnings about plagiarism plastered everywhere.
There are all sorts of ways an answer can be bad - trying to enumerate all of them just encourages folks to find loopholes, and prompts others to get all legalistic about it in an attempt to circumvent the system to remove posts they don't like.
There's a big arrow pointing down next to every answer. Use it.
And if you decide to leave comments, be constructive, avoid talking about voting, and resist getting drawn into extended discussion. State what you see as lacking or unhelpful, and move on.