If you're purelly seeding (as in starting to seed a torrent from scratch never having downloaded it from the bittorrent client you're using or having done it a long time ago - days, weeks or longer), without port-forwarding it will simply not work and nobody can connect to your machine and downloade anything for that torrent because all those remote machines that are trying to connect to your client have no association with your machine on the Mullvad Router doing NAT translation.

If you're downloading a torrent and then leave it seeding for a while after the download phase is over, then it will usually work fine because the Mullvad Router doing NAT Translation still remembers the various remote machines that your machine connected to in the swarm for that torrent during the download stage, hence when those remote machines connect back trying to themselves download stuff from yours, it will know that's related your machine and thus accept those remote connection and forward them to your machine.

In practice this means that it if you leave your torrents seeding AFTER DOWNLOADING is over, usually (but not always as for torrents with very few peers the swarm is either too small or changes too fast) you can upload more than you downloaded, hence you're not leeching.

So if you use Mullvad and don't want to be a leecher, always leave your torrents active and uploading after you've downloaded them.

Personally I have mine set to 1.5 upload to download ratio and only seldom does it fail to reach it.