Last time I checked pipewire was definitely not the default for Pi OS Lite since it makes no sense in that environment but maybe they’ve changed things around. I don’t think it’s your problem here, you’re not going anywhere near pipewire in your Mopidy setup since you’re outputting to a fifo.
The first hit on Google for your error message takes you to Gstreamer error - #13 by bachschmidmedia did you try it?