So, is it allowed to use sprites made before the jam starts? I'm gonna do PICO-8, and I was trying it out and made some nice player sprites. Am I allowed to use them on the jam?

Most jams ask that you do not work on game assets prior to the jam starting. I’d recommend not as it is generally unfair to the other participants. Plus, you don’t know what the themes are until the jam starts.

But by all means, get some practice in before the jam starts, especially if you’re using new tools or processes.

My own personal line on this is if the assets are CC0 and freely available to anyone else online then it's fair-game because everyone would have the same advantage.

In this case, I personally wouldn't use them because it gives only yourself an advantage.  Seems like it's counter to the spirit of the game jam.