As long as you don't start working on the game or creating assets for the jam before the theme is revealed, you can brainstorm as much as you want to. 👍
It depends on the kind of person you are though.
I know my best jam games were made when I didn't have a plan in mind before the theme was announced, and my worst were made when I had a plan in mind before the start of the jam and tried to cram the theme into my original idea. 😅