Mullvad got rid of port-forwarding so it's not good for torrenting anymore. IVPN, which is where the Mullvad refugees fled to, just got rid of it as well. I think AirVPN is the next option people are flocking to, but who knows when they'll do that next.
I've also seen stuff about torrenting over I2P which shouldn't have this limitation, but I'm not sure how usable it is right this moment.
For peers to connect to each other, at least one of the two connecting needs to be port-forwarded, so if everyone were on Mullvad, now no one would be able to reach each other. Right now you can still torrent on Mullvad, but only with (non-Mullvad) users who use port-forwarding.