Translation:Are you tired?
That is only a simplified way of explaining it. The rule is to enable the listener to perceive the magnitude of the adjective whenever that adjective refers to a progressive property. In a question and its direct response, such rule does not apply. The conversation can be:
For the first question both the question and answer does not require 很. For the second question, the issue in concern is how tired the listener is; the listener must answer with a magnitude of some kind, but you can see 2 of the sample replies I have given do not have 很 in it.
It has also to do with comfortable speaking. In the question, you have 3 characters that are comfy to pronounce (for Chinese): 你累吗？ If you only have 我 + verbal adjective in a sentence - that feels not comfy. 我很累。我很忙。我很好。 are more comfortable to pronounce than a two-character sentence like 我累。我忙。我好。 This is also why in Keith_APP's example sentence, there is 累啊！instead of only 累！ - the 啊 makes it more comfortable to pronounce.