Yes, it is natural in the right context. If someone justifies being idle, and you think the justification is not good enough, your sentence is natural indeed.

Thank you, Kenny. Yes, a good reason is fine.

It's not a reason for being idle >ok . That is not a good reason for (you) being idle ( you answered them when they reply to you with an answer that you cannot accept)). If you see them idling > You don't have to idle ( even if your work is finished ) . Go find something else to do If they answer you with a good reason > That is still not a good reason why you should idle You should use 'good reason' because their reply is telling you the reason . So, is it good or bad? Unless , their reply is totally unacceptable

No, I mean that if you don't have to do one of the necessary things, that's not a reason for not working hard to finish the work on time.