The term is - I find - misleading or misinterpreted, and I don't use it myself; but there is a sense in which we have 'fallen' from our pre-mortal, spirit state - passively dwelling 'in' the divine presence - into the imperfections and mixed joy and sufferings of incarnated mortality - en route to (if we choose it) incarnated immortality.
It seems that - presumably due to the constraints of reality - it is not possible for pre-mortal spirits to incarnate directly into immortal bodies. In other words, an immortal spirit cannot (certainly does not) become an immortal incarnate Being.
Therefore we must go-through this transitional state, in which we have a body - but this body is mortal, temporary; subject to continual change; subject to malfunction and disease; not fully controlled.
It is in this sense that we are 'fallen'. As pre-mortal spirits we were not subject to such problems; but as mortal we are.
Some individual people have very little experience of mortality; since they die in the womb or shortly after birth - indeed, this probably accounts for the majority of Men who have ever incarnated. But little experience is not none - all Men have experienced the essential nature of mortality.
You and I have a great deal of experience of mortality; and for us this has been a changing world experienced by changing mortal minds and bodies.
It seems that God has made the best and most of our mortal experiences - because in a world of continual change of this kind, we can get the maximum of experience: good and bad, beautiful and ugly, virtuous and sinful - according to the needs of each individual.
We can - if we learn - learn much from our mortal lives in this temporary world; en route to the permanence and much slower and lesser changes of Heavenly life in immortal bodies - bodies become self-regenerating, minds become permanent in memory.
Of course, some - perhaps many, do not choose resurrection and immortality; and for them this world may indeed be truly fallen from primal passive bliss - to which they hope to return. (Perhaps such persons have been mainly incarnated in the East, and follow religions such as Hinduism, Buddhism?)
But for those who follow Jesus, this world is well-designed, fit for purpose - indeed a world (albeit not 'perfectly' so, because such 'perfection' is neither possible nor wanted by God) specifically tailored around our primary personal needs.