The Joplin icon is going to change soon. The one we have now is something I put together quickly, not knowing if the project would interest someone, so I didn't want to spend too much time on it. Now that the project is more mature, it makes sense to start improving the visuals - first the icon, then the logo font, the website and finally the app UI (although these have already been improved little by little over the past year).
Before picking an icon, I'd be interested to hear about your feedback and whether you have a preference among those below. They all share the same idea - which is something that looks like a note, and that contains a "J" too.
Feedback is welcome! And if you have a preference please answer this post and put your top 2 or 3 icons in your post and we'll do a tally in a few days.