Nice, but ideally assets should be made during the jam, otherwise it could affect your rating.
This is true, but derivative assets would probably be fine… Also it’s up to the participant, they can just mention it in their submissions if they just don’t intend to be rated on Graphics or be slightly penalized in their score for using pre-made assets.
Assets is not always the end, having a good visual look sometimes depends on shaders, editing and what not, so I’d say it depends on how the assets are used.