This is not just one simple question.

You need to break it down into smaller tasks.

First things first, you need to get data from a Pixelfed instance, ok, start by looking up how to connect to the Pixelfed API.

Once you have a way to connect to the API, you need to request the data you need.

Once you have the data, look at the actual data, do you need to do extra formating?

Once you have the data, it is time to get/build the display.

Once you have the display, you need to figure out how to get your computer to talk to it, is it as simple as just sending the number to the serial port or do you need to use something like the GPIO interface on a Raspberry pi?