So my question is when I make a game for this jam should the game design be unexpected (controls, mechanics,...) or should unexpected things happen in the game like jumpscares (lol). Or maybe is it just up to us how we interpret this theme?
It can be anything unexpected! This could be an unexpected story, or unexpected game mechanics, or unexpected audio/visuals, or any other unexpected elements you can think of.
Overall you're trying to create an experience that the player doesn't expect.