As I understand it the essence of 'modern' thinking lies in its contrast to 'classical' thinking.
Classicists consider the past valuable in itself and in need of preservation, and either take some kind of Golden Age position or expect full integration of the important aspects of the past back into the present on an ongoing basis so that what was important is not lost.
Moderns consider the present more important than the past, and consider history valuable only to the degree that it shapes or explains the present. This implicitly presumes progress as a natural part of history. Otherwise reaching back into the forgotten past could improve the present at any moment, dispelling the notion that it is not equally important and should be consciously preserved.
From this most stripped-down notion of modernism as a concept, modern art is marked by its refusal to let its values be dictated by tradition, or negotiated in terms of our place in history, and attempting to derive them from internal sources or from our current mindset. Some of it is decidedly psychological, sociological or topically political, for instance, and constructed to have its effect via a given theoretical prediction or chosen current opinion. At the other extreme, some is focussed on being in the moment, free of other influences, and producing what speaks to the individual moment, and therefore to the present period.
So post-modernism, in its purposely oxymoronic construction, was initially a movement to consider older ways of thinking and modern ones more evenhandedly, without making this decision, and to re-mix the two. In particular, they often aim to reinject romanticism, primitive reaction, supernaturalism, the religiously numinous, the weight of anachronism, etc. into conscious awareness in the present to improve its depth, because late modern thinkers tend to report the impression that a scientifically-based society seems to lack depth of feeling or full immersion.
This requires directly rejecting the notion of overall progress and the unquestioned value of the scientific approach (which aims for progress), without overly romanticising or attempting to retrieve what is lost from the past without good reasons.
From that most basic version of the post-modern point of view, this is a misplaced emphasis, but not altogether bad. The modern moment is, after all, still a moment. But to insist upon it at the cost of being incomprehensible due to the vagueness of the context and the way right now lacks traction because it has not sunk into our awareness quickly becomes silly.