There isn't a single XMPP client for iOS that properly follows the iOS HIG and isn't ugly or unusable. There are very few clients that are even fully featured. The XMPP landscape on macOS is even more bleak.
The intersection of developers who want to develop on and for a proprietary walled platform and also want to work on open source clients for open standard, descentralised protocol must be pretty small.
If anyone reading this thread has iOS dev skills and cares about improving open-source messaging on iOS, feel free to get in touch.