Zim appears to depend on gtk, which is not supported/runnable in pythonista. So the GUI won't work. I notice Zim provides a web interface alternative, which might be workable via UI.Webview. zim.www has an import from zi.repository import GObject, which would be problematic, but doesn't seem to actually be used. I have not looked further into the code to see how much it depends on gtk, or whether you could truly run it without that dependencies.
Pythonista has its own GUI system, or you can display and use web based guis that interface back to python. So it would be theoretically possible to create a wrapper around the Zim GUI interface, while still using the underlying code and formats,etc, but just not clear to me how much work that would be.