Part of the difficulty in this thread is that the word 'mundane' seems to be used different ways. My preferred definition is
"
Very ordinary and therefore not interesting" or "
Being part of ordinary life and not special".
https://dictionary.cambridge.org/dictionary/english/mundane
That's what I mean when I use the word.
Then I can understand there might be some confusion when others use it differently. However...
The UAPs seem to me to be extra-mundane because they are extraordinary in the literal sense of extra-ordinary. They are events that seemed extraordinary to trained and experienced radar operators and jet pilots. They were events that fell well outside the range of their typical experience.
When talking about "mundane" they are usually talking about the general cause, not the phenomenon itself. So a remarkable weather formation would be mundane, as the cause is the weather (a mundane matter) even if the phenomenon itself might be extraordinary.
As such, using "mundane" in these contexts would fit with your understanding. What was the underlying cause of the UAP: birds? So "mundane", even if it was a rare formation that led to the interpretation of technological craft. Technical glitch? Also "mundane" even if the glitch was a one in a billion (glitches are common even if specific ones are rare).
This would also stand for the confluence of individually mundane matters into one extraordinary situation. To me, and others, this would be "mundane", as the underlying causes are common.
I take that very seriously since they are the experts in the military aviation field. (As opposed to armchair critics with no military aviation experience that I'm aware of.) In combat, lives will depend on their ability to distinguish enemy aircraft from birds and I don't expect that making that distinction is a big problem for them.
Yet surprisingly they do make those mistakes, as evidenced by the number of UAP sightings that have quite clearly been birds.
But there seems to be another usage of the word 'mundane' such that it means
"Belonging to this world, not heavenly"
and I get the impression that some of those who fight so violently against any suggestion that these events are extra-mundanc take it to be a claim that they are heavenly or extraterrestrial.
While I disagree with the implication that there are some here who fight "so violently" against such, I am one who would tend to take this view of the word rather than the former that you offered. The takeaway, though, is that one should probably be clearer (on both sides) what they mean by "mundane" when they use it. To avoid confusion.
That said, non-ET causes that I
wouldn't consider to be "mundane" would include secret military tech that is a step-change from what Joe Public would recognise. I'm not aware of any UAP cases that have been proven to be such, though.
So far, all UAPs that have been identified have been mundane in origin, and the only
extraordinary thing about them in some cases is how people, many of whom should know better, interpreted the observation so wide of the mark.