Sorry, I'm not following. With e.g. HTTPS traffic all I have to do is make sure that the browser sends all its traffic through 127.0.0.1:8080 and trusts mitmproxy's certificate. In that case, mitmproxy will show me the plaintext traffic.

Now, let's say I've set up WireGuard with three peers, i.e. each of these peer's public key appears in my WireGuard config along with an IP address. Now I start sending traffic to one of these IP addresses which WG would normally encrypt and then route directly to the peer in question. How can mitmproxy's WG mode help me here with "getting [the encrypted] traffic into mitmproxy", as you say, and reading the plaintext?