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.

I did a quick bit of research and port forwarding allows for faster connections to peers, meaning you get faster downloads and should reach seed ratios quicker too. Apparently you need port forwarding for private trackers too.