The Notebooks/Documents/Content panels in Joplin used on iPadOS are mirrors of what is used in iOS which works great on a mobile phone screen size but annoying on iPadOS in landscape mode.
Using the panel layout from MacOS would make Joplin much more usable on a landscape iPad. This improvement should also take advantage of the iPadOS 16 option to expand usable screen space, making it feasible to have all 3 panels on-screen at the same time.