I can try give some general advice, as I don’t know the specifics of Pixelfed. Presumably it has an API like Lemmy, as third party apps seem to function, so you should check the documentation for getting the number of likes on a post with the API, and write a script to display it on a monitor as text, as it’s unlikely such a script exists already, although it might. If you’re unsure how to code it, I can try help if you need.