OK, so I think I figured it out, leaving this here as a little how-to:

There are two options for emulating Pico-8, Fake8 (default), and the “stock” Pico-8 Software.

To use Fake-8, the simpler emulator, just put .p8 files in your /Roms/PICO/ folder and run them as you would any other game. You need the cartridge files however, so if you want to run games published on e.g. itch.io, then you need to make sure the developer has made those available. Devs are often really kind and make them available on request for downloading.

Using the stock Pico-8 software will be the more exciting part because it will give you access to Splore, where you can try Pico-8 games posted on the official website.

To enable the stock Pico-8 you need to follow these steps:

1. Purchase the Pico-8 engine here: https://www.lexaloffle.com/pico-8.php.
2. When your Humblebundle download link arrives in an e-mail, go ahead and download the raspi files under the Linux platform tab. You will need to put some of these files on your SD card.
3. On the Miyoo Mini enable the original Pico 8 engine - Apps > Package Manager > Expert > enable “PICO-8 (PICO-8 Standalone)”, then press <START> to save the settings.
4. On the Miyoo Mini enable “Expert mode” - Go into Apps > Tweaks > Appearance > toggle “Show Expert Mode”

Now you should have an Expert Mode app on your front screen, with a PICO-8 app inside. When you open the app, it will give you your Roms listed in /Roms/PICO, but a new option should appear there: ** “-Run PICO-8 with Splore”.** If you run it now, it will tell you to put a few files on your SD Card:

… COPY “PICO8_DYN” AND “PICO8.DAT” TO YOUR SD CARD IN /RAPP/PICO-8/BIN"

So do that. Works like a charm! Just make sure you have Wi-Fi connected.