Rule creation interface is hard to use
(I am filing this here because there does not seem to be a component at https://bugs.kde.org/ for this project yet.)
Looking at the code here and the screenshot under https://www.omgubuntu.co.uk/2021/01/kde-plasma-5-21-features , it looks like the rule creation interface was taken almost unmodified from the predecessor nx-firewall (formerly nomad-firewall). Unfortunately, this interface is unnecessarily hard to use.
UFW-KDE, the old KDE 4 KCM from upstream UFW, had a "simple" mode (which was the default) and an "advanced" mode (easily enabled through a dropdown at the top). In the "simple" mode, only the destination port (for an incoming rule) was shown (with the possibility to pick an application profile instead of a port number, see also #14 (closed)), all the other settings (source address, source port, destination address) were hidden and set to "any". In the "advanced" mode, everything was shown. Plasma Firewall currently only has the equivalent of the "advanced" mode.
The rationale there is (and I tend to agree with whomever came up with that design) that an inexperienced user will not know that the destination port is the only setting that really matters in nearly all cases and what to do with the other settings. The "simple" mode allows such users to just open up a port (or ideally an application profile, see #14 (closed)) without knowing the technical details. And if you need to set everything, you can quickly switch to "advanced" mode.
I think Plasma Firewall would greatly benefit from having such a "simple" mode, or something equivalent (could also be, e.g., a separate "Add simple rule" or "Quickly add rule" button), available as well.
Of course, if you are editing a rule that uses advanced settings, it should automatically come up in "advanced" mode so that the non-default settings are clearly visible.
Somewhat related to this, the defaults when creating a new rule should probably be Incoming and Allow, at least if the firewall is set to default deny.