Since many people have just gotten their hands on the playdate for the first time, they are probably prototyping the game they've had brewing in their head for weeks/months/years right now. It might not be the right time for a broad game jam.
If you narrow the focus to, say, microgames, there might be more interest.
There has been much discussion and a few efforts to get the ball rolling on this already, but they were a bit premature as they started before the SDK went out.
- Thread from 2020-08 by @Nic
- Reddit discussion (2021-08)
- Reddit discussion, prototype (2021-12)
- @Drew-Lo discusses this on Discord: (1), (2)
The key to this project, I think, would be (a) a good API so that it's easy to contribute minigames, and (b) to research and consolidate the efforts of those who have suggested or made strides toward this in the past.