I generally agree that somebody being completely stunned feels like a Taken Out result rather than a Create Advantage - after all, someone completely paralyzed is pretty much out of the conflict, right? If you think of most movies, TV shows, or books, the "paralyzing attack" is generally something that occurs at the end of the fight, not the beginning.
That said, in addition to the other answer, there's a number of things that you can do to get the flavor you want.
One of the common effects that an aspect can have is to provide passive opposition. So being Stunned might provide a passive opposition of +2 or whatever to various actions that the stunned character makes.
In practice, this ends up being as a floor for the roll of the other character when active opposition is being used. So if you're Stunned and attacking me, then I know that no matter how badly I roll on my defense, I always have a minimum of a defense roll of 2.
In a recent game I ran, I had ghoul-like undead who had typical D&D-esque paralyzation. I handled this exactly in that way. A Create Advantage that laid a Paralyzed effect slowed down the PCs, and provided passive opposition to nearly any physical action.
I personally prefer passive opposition to handing out bonuses, as bonuses can degrade into either having things that can't succeed/fail, or having people arguing for justifications for bonuses for every single aspect or fact in play. YMMV, of course.
Permissions and Denials
Actions take place in the fiction of the world before they're translated to rules. One of the things that being Stunned can do is to make certain actions simply impossible for the Stunned character, or conversely to make certain actions possible that would otherwise be impossible.
Probably the best example for narrative denial of action would be Spider-Man webbing someone. They can break out of the web, and there's things they can do, so they're clearly not Taken Out, but while they're in the web, there's a number of things they can't do. For instance, they can't go somewhere else, and you could make a good argument that they can't dodge out of the way of anything (at the very least, they'd have quite a bit of passive opposition).
A different type of permission actually goes back to the "attack" in the first place. Having a certain technique might allow a character to attack certain opponents that otherwise would not be attackable. You might have an NPC with a Toughened Skin aspect that made it so that most normal punches/kicks did nothing to him. However, in that case, a Paralyzing Nerve Attack would likely bypass it.