No, save polish for last. Work on gameplay mainly. Trying to make a visually appealing game and not focusing on the gameplay will make the project fail. Save visuals for last, gameplay is the most important part of a jam game. This is my 7th jam and I've been using Unity for a year but during my first jam, I made these same mistakes. Also, it doesn't need to be story focused. There are many types of games and I would NOT recommend story focused games to a beginner.
If you need any help, feel free to ask me.