I think that this kind of Game Jam is going to help you a lot. You don't need to be the best or good in anyway, just have fun! Try to finish the game and have fun with it, no need to overwork and learn to manage time frame is a key here.

Also, if you try to make a game. Make a SIMPLE one for your first one. You will learn so much?  A game like Snake, Concentration, Space Invaders, and so on. One screen, simple player controls, simple movement.  Just get a simple game DONE! It will feel so great to get it finished. I made 50 little games and demos before I got in the industry. Gotta learn your craft.  It's okay that it is simple. Getting games done is the hardest thing to do, so give yourself a break and make the first one easy to make! Good luck!

You can join most jams even if you aren't that good tbh. Some people might take it seriously and competitively, but many just join for fun. Just remember to get inspired by the other more skilled jammers rather than get discouraged. 

Good luck, and remember to have fun! 

As a game dev you'll constantly be learning so don't stress on not being too good. I'm not either - but I find the limitations inspire me and force me to think more constructively. Start with a simple game with simplistic controls - but you'll still be amazed at what you can make! And it feels very fulfilling just completing a few projects. Don't sweat it - just apply yourself, learn and you'll create something special, at least to you if not anyone else :)