1. You can make assets before the jam starts, but you are required to disclose which assets were premade before the jam in a doc you will submit. Unless the entire game is pre-made assets/other people's assets your game's score won't be affected much.
2. MelonJam has a theme we only release at the start of the jam to prevent people from starting on the game beforehand. The theme score is weighted pretty heavily, and so having a game that connects well to the theme is very important
Hope that answers your questions!