IIRC the idea was to seriously Impervious her - if your Damage Rank isn't high enough you're never going to hurt her even by finessing around - there are no weak spots to hit (the assumption being that if there were a weakness, the Widow would have used it).
However, critical hits are more than just "weak points." They are also "rolling doubles," i.e. they could also represent momentum, doing structural damage to fairly homogenous things, etc.
But right now I can't check whether Criticals and Power Attacks are meant to raise the "base" Rank of Damage when facing an Impervious Defence.
Impervious specifically compares to save DC modifier, not rank, and hence a good roll, or Power Attack, can overcome it. In DC Heroes, the default strategy for being unable to breach the armor would be, "Forget about Critical Strikes, just spend HPs and hope for a double." In M&M, the equivalent would be vulnerability to criticals, but perhaps some resistance to Power Attack. However, a higher Impervious rating by itself would actually encourage power attacking. It probably does make sense for Impervious to exist, since her armor is clearly unbreachable by normal firearms.
I would suggest making her armor Impervious all the way up. Compare these two constructs:
Immunity 1 (bonus damage from Power Attacks), Limited (takes half effect) @ 1 point
Protection 2, Limited 3 (only versus Power Attacks) @ 1 point
Both of these constructions inflict a different sort of "cost" to power attacking, while Impervious protection makes it a necessary strategy to do straightforward damage barring a critical hit. That sounds right. The second construction means that no one without the Power Attack advantage is going to derive any benefit from trying to Power Attack; if that seems harsh, it could be reduced to Protection 1.