It's not about performance, of course. It's about looking like HTTPS, being impenetrable, separating the ad-hoc transport encryption and the Wireguard encryption which also works as authentication between endpoints, and also not being not TCP inside TCP.
TCP over Wireguard is two state machines stacked on each other. QUIC over Wireguard is the same thing. Yet, both seems to work pretty well.
I think I see your argument, in that it's similar to what sshuttle does to eliminate TCP over TCP through ssh. sshuttle doesn't prevent HOL blocking though.
TCP over WireGuard is unavoidable because that's the whole point of tunneling. But TCP over WireGuard over QUIC just doesn't make any sense, neither from performance nor from security perspective. Not to mention that with every additional tunneling layer you need to reduce the MTU (which is already a very restricted sub-1500 value without tunneling) of all inner tunnels.
You can just do that by using QUIC-based tunneling directly instead of using WireGuard-over-QUIC and basically stacking 2 state machines on top of one another.