i am not good at pixel art or 3d modelling so i want to use free assets from the unreal marketplace, is that allowed?

What assets can I use?

- You may use free and open source assets as long as you are legally allowed to use said assets. There is strong preference for what is created during the jam. Working within the bounds of what you can do is a wonderful breeding ground for something truly unique and interesting.

So, yes as long as the assets are free and open source and you can legally include them in your github repo.