I got an idea of what I want to make for the Jam but it involves my pre-existing fictional world I made for my TTRPG sessions. would it be considered cheating to use this setting that already exists on my Obsidian Vault to kickstart my brainstorming of my entry? I can always make a new world for this project if it is I just need to know before committing to it.

Using your pre-existing fictional world as inspiration for your game is completely fine. The key rule is that the actual game itself must be developed during the jam period.