I hope this isn't too controversial?
The "name" section is what the browser debugger will show, and it of course knows nothing about the Itanium mangling conventions - it doesn't even know that the Wasm file came from C++. So the demangled names should go in there?
I seem to remember a GitHub comment, perhaps from @sunfish or someone else saying that this had been the intention - but maybe I'm misremembering.
Alternatively, DWARF-like debugging data attached to the Wasm file could play a similar role, but then, what's the point of the "name" section if the demangled name for the user is actually going to be coming from somewhere else?