My game uses a lot of text, and I was using a default system font for my game. I switched this out for a font I found online that fits the theme better. Is this allowed or do fonts count as assets that must be made during the jam?

I found this post from GBJAM 5, so I will assume its precedent of allowing this will still hold

https://itch.io/jam/gbjam-5/topic/41667/fonts

Yes, it's allowed. There's an FAQ in the main page so you can check that first if you have any more questions