The one Highsmith novel I read had uniformly unsympathetic characters. The murderer was the least crazy and most comprehensible one. It wasn't bad, exactly, but I just didn't enjoy reading about any of those people.
None of her characters are likeable, and the sociopath as "relatable hero" is a common theme. That's a Highsmith thing. But even among her books, the characters in this one were particularly dismal.