The mount terminal command should work. But I’d expect you should be able to right-click an .iso file and mount it that way too. I’m not on Linux Mint but can confirm with GNOME on Debian it’s easy enough to right-click the .iso file and select “Open With Disk Image Mounter”, Cinnamon on Linux Mint should be able to do the same or similar.

Also keep mind you’re probably going to need to do more than just mount a .iso disc image if you want to play an old Windows game on Linux. There’s a few ways to go about that but I’m not too familiar on the best approach when you’re not running Windows games through Steam.