The github code handles talking to a web service (The Cat API), retrieving a random cat picture, scaling and dithering/quantizing it to greyscale for the MagTag, and then feeding the MagTag the image in a sort of super stupid bitmap format.
To use:
Get an API token for The Cat API.
Download a release
Run it somewhere MagTag can access with -api-token=YOUR-TOKEN
Edit magcat.py to update the URL (keep the /raw at the end!), save it and the contents of lib/ as code.py to a UF2-using MagTag