Hi,
this is indeed an interesting question, since it may vary from case to case.
I would not judge by the number of games one has made, because if they are just learning projects then you might still qualify as a beginner.
Of course "not exaclty great" is also subjective.
If you see the gamejam theme and you can create a game without any struggles that also looks great and is well engineered than I would not say you are a beginner. This would however not apply if you switch technology. For example I can create 2D games in pure C++ very fast, I have also created my own C++ custom game engines and published a few games. But if I would now switch to UE or Unity I would need to learn new and different stuff, so in that situation I would qualify as a "beginner".
It is only natural that we have people here with very different level of progress.
But from what I can see from your profile (some small games), I would say feel free to join the gamejam and have fun and make new friends. :)
Hope this helps
Kind regards