I've said it before and I'll say it as many times as necessary...
Rule Number One of Gun Fighting: Have a gun.
Sure, not all firearms are great for Self-Defense, but most are at least adequate given that, statistically speaking, most would-be attackers will cease all hostility at the mere sight of a gun, and the vast majority of those who don't will once they've been shot.
Those meth-heads who take several shots and keep on coming are the exception, not the rule, hence why they make headlines in the first place, because ordinarily criminals (including those who abuse narcotics, contrary to popular belief) are just looking for an easy score, somebody who will hand over their wallet without putting up a fight, much less drawing a potentially deadly weapon.
Is .17HMR potentially incapacitating? I have no idea, but considering that solid .22LR bullets fired from a pistol are more than capable of penetrating deeply enough to damage the central nervous system, I don't see why .17HMR couldn't do the same.
Regardless, it shouldn't be anybody's first choice for Self-Defense, but you use what you've got, and if that happens to be an NAA Mini Revolver chambered in .17HMR of all things for whatever reason and you have to use it in fend off an attacker, perhaps while on your way to the Gun Shop to exchange it for a more suitable Self-Defense firearm, go for it.
Oh, and let us know how it goes afterwards!