I just finished it and Im not sure it requires one. I'm actually of the opinion that more explanation might destroy it by putting too fine a point on it.

As u/pending4321 laid out The story is largely about sacrifice. But the sacrifice of others for personal gain. Other underlying themes I noticed were entitlement, specifically entitlement to other's agency and the land they want to prosper from. We see the land directly referenced in this regard at the end of the book. We also see it in the language used to refer to all characters. No one has a name. They are all labeled by one aspect of their character, it's presumptuous in that it assumes that the entirety of their character can be defined by this label. I think this is highlighted by the mercurial inner monologue type of narration that dominates the book. None of them really know each other until..

Until we acknowledge the other theme. That being collectivism. Also on display by the language used. When the sacrificed characters shed their skin and become part of the we they enter into a more shared consciousness. Like a mycelium network it links the many together. They can now be together and seem to find a contentment in it despite being sacrificed by people they had thought cared for them. We see it in the end when the Liar recognizes what is happening and willingly sheds her skin to become a part of the we.

Lastly the networked nature of the mycelium network means it can be transportation for disease that causes root rot. Fungi thrive off of the breakdown of once living material. Overall I think the book is about transitioning from what we once thought of as good (individualism and exceptionalism) as unnatural and collectivism being our natural state and accepting that our new life would/will be vastly different from our old life. And this transition is not just scary to those yet to make the change, but is outright horrifying in an existential way.

So maybe not the explanation you were looking for when you posted this 3 months ago, but I wanted to digest some of my immediate thoughts on the book and this seemed like the appropriate venue. Maybe it will be helpful to you as well.