I feel like the disapprovals and approvals of Shadowheart fit the fact that she is on one side a good character, and likes when something good, or avoiding unnecessary bloodshed is made. But on the other hand, she sees herself as Shar's disciple,
and as she says at the party, Shar wouldn't approve of helping the tieflings.
So, the way I think is that if you do or say you will do a good deed she will disapprove because she's conflicted and wants to show Shar that she disapproves of those good acts. However, if you deny the direct answer as like, I'm gonna save/help and instead use evasive dialogues and do the good action, usually she will approve in the end or not disapprove.
This at least is how I see her. On one side, she's sweet and actually very pleasant. On the other, she's the most annoying and infuriating character to talk to when she starts talking about Shar.
Also, what I feel lacking for now is that she doesn't show much appreciation for the player after the night at the party, and return to the evasive Shar disciple. I think we'll get a better result on her approvals later acts when we can take her away from Shar and make her a Selune follower.
