ADHD alone is estimated to be a disorder present in 5% of the adult population (2% are currently diagnosed; the 5% is based off childhood diagnosis rates and the fact it usually doesn't go away when becoming an adult).
And while those with ADHD are unlikely to populate the OP's 5%, those with pathological narcissistic, sociopathic, schizophrenic (and other) disorders very well could.
I sort of think what you're really seeing within that 5% is a majority fraction of "baseline shitty" people, and a minority people of "momentarily shitty" people.
The baseline shitty people are likely a kaleidoscope/patchwork of pathologies, and the momentarily shitty people fall within normal ranges but circumstances have pushed their stress levels north of what they can handle.
Is that to say that people don't have personalities which suck? Of course they do. That being said, there are many ways of balancing a person's tendencies towards negative outcomes, but ONLY if they can admit they have a problem and they're willing to put in the effort to re-balancing themselves.
A lot of hyper-toxic fiddly freakouts over small things actually turn out to be deeply rooted in some sort of horrible trauma.
There just isn't a large enough population of mentally ill with symptoms that would manifest as described to make up 5%, though they surely make up some part of it.