“All bugs are not created equal” would definitely seem to be a major factor in raw “counting metrics” like # of bugs closed decreasing over some period of time.
One other factor to consider as well is, as Plasma matures - especially working through the growing pains of adapting to Qt6 and all the upstream Qt bugs and downstream packaging bugs that have cropped up - I think one would expect that the number of bugs being created would also decline.
That does appear to be trending to be the case since the Megarelease, with spikes that do occur in bug creation trending along with distribution releases:
(sourced by pulling Bug List for each month - November is of course partial but would extrapolate so far to under 400)
So even if all else were equal, there would be some degree to which fewer bugs going in would mean that there are fewer bugs that can be closed.