One of the ideas I see in Daoism that is conveyed so well in Mushishi is that of a world not characterized by absolute peace and harmony but by imperfection, by the grotesque and the disturbing and the painful, and of the importance of finding our peace in this world not by avoiding or erasing those imperfections but by accepting their inevitability. People who are told that Mushishi is beautiful and soothing are sometimes surprised to discover how many dreadful things happen in it, and amazed at its ability to leave behind a feeling of serenity despite them. I think this is part of its magic, and also part of its teaching: that pain and conflict are necessary parts of life and the world, and the path to peace is to accept that; that not everything needs to be okay, and we can move on even if it isn't; that if the world ever did become perfect there would be no more need for change, for movement, for life, and to accept life is therefore to accept the inevitability of imperfection. Problems will occur, forces will conflict, people will make mistakes, pain will come and go. The fact that mushi can cause problems doesn't invalidate their existence; the fact that humans screw up doesn't invalidate theirs; the fact that life involves pain doesn't make it not worth living. The end of each episode of Mushishi is a reminder of this: that even if imperfection remains and will remain, we can still return to peace.
The only thing I would add to this is that the world is neither characterized by absolute peace and harmony nor by its imperfections. That the world is inherently good and beautiful even despite the imperfections that exist. I think that's an important belief that balances all of this. And I think you can see this in Mushishi as well, if you look for it. Definitely one of the best animes ever.
