That’s the default DE you’re talking about.
KDE is officially supported as far as the docs say: KDE (desktop environment) | Qubes OS
More over, there’s a tendency by Qubes to support KDE 6 and to move towards wayland as x11 will be dropped by Fedora.
No, I am talking about support of the KDE. I like KDE way more than XFCE or other GTK/Gnome ugliness.
Of cause user can install in dom0 any DE, it’s Fedora atfer all, but it will not make Qubes OS support all possible DEs. The tendency with KDE in Qubes OS is quite clear: KDE is not supported, not tested enough by devs (because almost nobody uses it among them?). Also, when KDE was an option during installation (not even default one), it was supported, because more more casual users were able to chose it during installation.
Nowadays, unfortunately, KDE in Qubes OS is not expected to work even as “fine” as XFCE, which itself is full of bugs and is lacking basic features like proper windows placement. Hopefully inevitable migration to wayland will force Qubes OS to move back to functional and advanced DE, because XFCE is half-dead and not actively developed. But currently we are not there yet.
But this all is off-topic.
I think XFCE people are working on it to make wayland compatible.
That would be unfortunate and would prolong xfce-era of Qubes OS with ugly look that scares away casual Linux users and lack of basic DE features that annoys advanced users.
Why not list these bugs in the corresponding thread then, perhaps, so we could discuss it and maybe help you out
https://forum.qubes-os.org/t/kde-changing-the-way-you-use-qubes/