I wrote a script to show search results from Joplin in the GNOME Shell overview. You can find it here. It requires Python to run plus a few deps (including the nice joplin-api library) and Meson to install.
Here's a screenshot!
There are two known issues:
It's not yet possible to open a specific note in Joplin. I suggested adding a commandline option and discovered the plan to add x-callback-url support, which will be great once there's a design for how it should work.
The search provider appears as an application in the Shell. I think a change in GNOME Shell is needed before we can hide this.
Let me know if you like it!