I'm tempted to write Dan to let him know that some people (well at least this one) were confused by the previous comic even after reading the commentary and following the links. (Incidentally it's very strange to be in the "confused by the comic because I forgot things from previous strips" camp; I guess I need to re-read the last few years of the comic.)
I'm glad the comic points out why Grace was so distracted, though I still think it would have been better for those of us reading it as it comes out if it had been explained in the previous comic or commentary (it won't be much of an issue for those reading it in the archives). (I also still wish Dan wasn't jumping around in the timeline so much.)
As for Grace's goodness, Justin wouldn't be so surprised she doesn't see herself that way if he knew about how she reacted to her part in Damien's death. Of course her feeling guilt over a foe's death is a sign of her goodness, but it also means her assessment of her own goodness will not be as high as that of an outside observer. And while the most dramatic instance, that was hardly an isolated incident; Grace is well aware she is a flawed individual, and the flaws she sees in herself weigh heavily on her.
Incidentally, I hope Justin was just kidding about Grace defending her "title". For one thing, bragging about being the most good individual around is not the sort of thing one would expect the most good individual around to do. For another, when used seriously "goody-two-shoes" is an insult, usually directed at those perceived as self-righteous - not exactly a title most people would want to fight for.