The other comments almost got it right. If you had your torrent client bound to Mullvad and then opened your Tor Browser… your torrent client would be running over the VPN tunnel (Mullvad) while your Tor Browser would be sending all its traffic over your vanilla ISP and through… the Tor network (unless you also bind it to Mullvad). You’d effectively be “split tunnelling” your traffic, which is actually a good use-case for Tor anyhow.

There’s a lot of debate about whether it’s fine to run a VPN tunnel (OS-wide) before you fire up your Tor Browser… effectively you’d be pushing your Tor traffic through the tunnel to the VPN’s entry/exit nodes before it got to/left the Tor network. Some say it’s a security risk (if you don’t trust the VPN provider, for instance. Which is valid if you’re using some of the scummier providers). You need to do some research and understand the implications of doing that, before just mashing buttons.

You can also fire up the Tor network system-wide if you’re crafty and then create an encrypted VPN tunnel to go over that, so all of your VPN traffic would be travelling over and through Tor nodes before it reached the entry/exit nodes of your VPN. It can work both ways. There are cases for both options, if you know what you’re doing… which is a huge caveat.

Overall though, no. Please don’t torrent over Tor. As you say, it’s not necessary and eats bandwidth from an already slow network protocol. A VPN is more than sufficient for that purpose. If you wanna get more secure than that, make sure you’re running an encrypted DNS solution (or resolve your DNS locally if you know how to do that) and profit. Then your ISP can’t see shit. They’ll still probably traffic-shape and throttle you, simply because they can tell it’s going out over an encrypted tunnel of some kind… but they’ll never be able to see what specifically you’re up to.