I don't know if this is the entire reason why it fails for you, but it looks like you forgot to escape special characters from the shell. Specifically, the & in the URL is a special character to the shell: it means "run the preceding command in the background instead of the foreground." Which means that you ran the command mpv https://www.youtube.com/watch?v=Vn7kB0f16jg in the background (which, as shown in your output, was assigned the process id 12138), and the command list=PL-2mqY82TEri1eOFZmhfwuLxX11V1TkcI in the foreground, which isn't what you meant to do.
Enclose the URL in quotes so that the shell doesn't interpret the & in a special way:
mpv 'https://www.youtube.com/watch?v=Vn7kB0f16jg&list=PL-2mqY82TEri1eOFZmhfwuLxX11V1TkcI'
I also wanted to point out that we do have a port for youtube-dl, and that if you installed the mpv port, it already declares a dependency on the youtube-dl port, so you don't need to (and shouldn't) install youtube-dl manually in /usr/local since MacPorts already installed it for you in /opt/local.
