So I was able to get PICO-8 Working using the latest version of retroarch and the Retro8 core. It is based on an older version so some newer PICO-8 games may not play.
After doing i lot of head scratching I found this post online which resolved the problem.
Make sure your Pico-8 files are .p8.png extension.
Install the Retro8 core. Place your files into a folder for Pico-8 in your games folder.
Turn off the Retroarch image viewer (User Interface-->File Browser-->Use Built-in Image Viewer)
Run a manual scan with Content Directory (Where your Pico-8 files are), Custom System Name (Pico-8), Default Core (Retro8), File Extension (.png).
Test each file you have. Since Retro8 is built off the 0.1b and hasn't been updated for 2+ years (as of 2024), some games that have been created after then don't work, or don't work properly (many sound and image problems). You will need to check each one.
As of this writing I have tested Celeste, Dusk Child, Just One Boss, Mai Chan's Sweet Buns, Pico Racer, Swordfish, and Tiny Ronin. While most work perfectly, I know Just One Boss is a bit off but playable and Pico Racer seems undertuned.
