Why Jimm cannot simply be ported to iPhone
Jimm is a Java ME (JAR) application, and iOS does not support running JAR files or third-party Java emulators through the App Store the way Android does. That means the client cannot be ported as-is — a separate native app with the same behavior has to be built from scratch.
What's already available on iPhone today
While the iOS version is in development, iPhone users can use the web widget on https://jimm.online directly in Safari — no installation required, with UIN login, a contact list, and chat. It is not a full replacement for a native app, but it is a working way to stay connected from your phone.