That sounds strange given that Mullvad works fine for torrenting in my personal experience and even up to quite a good speed (it can use the full 200Mbps download speed from my ISP)
Also modern NAT will do deep packet inspection on common well known protocols to automatically adjust the port of your machine listed on any "here I am" protocol messages being sent out from your side to be an actual port on the VPN Router and to have an internal association of that port in the Router with the actual port in your machine so that connections of that port can be sent to your own machine and the actual port in it that are used.
It's only the pure listenner services (such as webservers and e-mail servers) were the port is pre-defined by convention and not a variable one sent out on any "here I am message" that require explicitly configured port-forwarding on the VPN Router side, plus because the port is fixed by convention for each type of service (such as port 25 for SMTP and port 80 for HTTP), off all the clients connected by VPN to that VPN Router at any one time, only 1 will be able to get that specific port.