Except noting indicates to new players shadowheart is evil, especially when most of them won't even know who Shar is.
Shadowheart pops approval for most selfless acts and starts opening up to you pretty quickly.
Which is itself a flaw with how the game presents her story. Anyone from Faerun should know Shar is not a good being. However, nobody but the priestess that took her memories should know off the bat that Shadowheart may not be a reliable narrator about her own self.
Also, Shadowheart disapproves of a fair few selfless acts too. Mainly she approves of avoiding conflict because it wastes time, but she doesn't like going out of her way to help people either as that also slows her down. There are a few exceptions there. Mostly related to children. But it is her general rule.
But that is also why I don't consider her evil either. I see Shadowheart as more Lawful Neutral than anything. But as far as Shadowheart herself knows she's on a mission for an evil god and is totally fine with that. She even goes so far as to hate on Selune left and right. She's ride or die for her evil religion as of Act 1. So for me, she leans closer to evil than she does good at the point in the game that we meet her.
Which isn't to say she can't become good later. But for Act 1 I'd fine it hard to justify a good aligned character romancing a Shar priestess outside of an "opposites attract" story.
