The answer depends on whether you take 'normal' to be the world of public discourse in The West over the past century or so; or the other billions of people who live elsewhere, and lived in earlier eras.
Some people who have believed in demons were indeed psychotic - a tiny minority. By contrast, viewed biologically, many/ most of the people who disbelieve in demons are actively and aggressively deluded about many basic and important matters of common sense and human consensus - and are engaged in grossly biologically-maladaptive behaviour (e.g. they avoid reproduction.)
On this basis, to disbelieve in the reality of demons is a sign of being objectively crazy; although not by itself conclusive evidence.
The primary question is whether demons are real - whether they actually exist?
If you are a Christian, that question is pretty much answered for you; since demons are regarded as true in scripture, church tradition and official doctrine - although you would probably want/ need to confirm this for yourself by revelation and/ or direct knowledge.
As is usual in this world; there is broad agreement that demons are real, but a great deal of dispute about what they are. 'Evil spirits' - or spirits of evil intent... probably that is as far as consensus goes. Detailed understanding of demons will depend on the metaphysical assumptions of a specific theology.
A further question - unclear to modern Westerners - is what function demons perform, or might perform, in our thinking; the matter of what demons are necessary (or useful, or wise) to explain? Why do we need to acknowledge the reality of demons?
I can think of at least two important reasons. The first is that demons are assumed to be immortal - or at least much longer-lived than Men; and this can explain long-term strategic trends towards evil, purposive extending over many human generations.
By contrast, evil humans are often extremely short-termist and selfish; making it unusual for there to be long-term cooperation among evil humans to pursue evil objectives.
Secondly there is the actual nature of evil. Demons are engaged in spiritual warfare over the fate of Men; that is, they seek the damnation of Men - they seek that Men will, by choice, reject the offer of salvation and the chance to dwell in Heaven in immortal and resurrected form.
This is a different aim from human evil (except insofar as Men are serving the demonic agenda); because Men typically engage in material, not spiritual warfare; and seek material, not spiritual gain - they seek money, sexual gratification, power, worldly prestige and so forth.
Recognising that there is a spiritual war ongoing in the world can (it seems to me) make simple sense when it comes to understanding the apparent contradictions in evil as we observe it in the world. (An example is here.)
Finally, and only at this later stage, is the question of whether some specific demon is active doing some specific and identifiable thing in question: Is Dr X currently possessed-by a demon and serving as a mere-mouthpiece? Is Mrs Y consciously serving the demonic agenda, as she understands it? Is Committee Z being duped by demons? etc.
We would seldom if ever expect to be sure about such specific matters; even as we acknowledge their possibility.
But we need to be able and prepared to make a judgement; to decide and act on the decision - sometimes and under certain conditions; the decision that Yes: this is probably what is happening, here and now.
