For my jam games I kind of do things a bit backwards to the normally recommended approach, I create, then build, then tidy with any time I have left.
I normally go into a jam knowing 80% of what I want to make, and try to keep the scope limited to reduce the need to spend the entire time on asset creation.
The other 20% will be things learned in doing the jam, minor rescoping, or minor improvememts from incremental playtests.