Though one can dislike a person's actions and yet still have the intuitive cognition that they're actually a good person who may have simply done something that one doesn't agree with. And one can also not be fooled by a person's outwardly friendly, supportive or even charitable behavior because of an intuitive cognition that they're not really as they appear and that their actions may indeed have ulterior and perhaps even sinister motives. So, in this case, even though it might be somewhat understandable, I tend to agree with Roland that it really is quite ironic that Maggie's perceptions are so skewed - especially when it comes to Stokes, whom she already knew to be someone who is helpful and kind.
I suppose it's a case where Maggie allows her own anger towards Adam to color and cloud her perception of Stokes, and where she allows Nicholas' charms and generosity to overpower the inner voices that should be telling her to run the other way whenever he's around. But then, if she had listened and done the latter, the end to Nicholas in this particular plot wouldn't have been half as satisfying as it turns out to be...