WebRTC isn’t really a big concern. The maintainer of the Arkenfox project who has also worked with the Tor Browser, has stated the following regarding WebRTC on Mullvad Browser:
“You do not need to disable webRTC, as it is already protected by mDNS and is gated behind user gestures - the user must also explicitly opt in per site, per session to start sharing. WebRTC support is trivial to fingerprint.”