Most, yes, but not each and every one. Brunt, for example, seemed a relatively decent guy in the Mirror universe. I thought he seemed nicer than prime Brunt, even though he was a mercenary. Smiley or Jennifer Sisko didn't seem too bad either once you got to know them, their life just was a lot rougher to make them turn out the way they did.
Also, if it were true, what would Mirror Hitler and Mirror Stalin have been like? Come to think of it, we never saw Mirror Dukat either, did we?
I think at least the (weird) idea is that somehow at heart they are exactly the same people, bent by different circumstances. In Mirror Mirror we see that Mirror Spock is not so different to Prime Spock.
Like, being from the MU doesn’t inherently make you a bad guy. You could even be a good guy in both.
I suppose in many ways every subsequent writer has been getting it wrong in terms of what Mirror Mirror portrayed originally.
I don’t know exactly though.
Controversial Opinion 1:
Star Trek should never go to the MU again.
Controversial Opinion 2:
Unless it’s for Kelvin 4.