As it happens, I agree that this is not Highsmith's best - that would be The Cry of the Owl, or This Sweet Sickness, or Deep Water, or maybe one of a few others. However this reviewer seems to be making the mistake of judging a work of fiction on moral grounds. Weren't they listening when Oscar Wilde said, "There is no such thing as a moral or immoral book. Books are well written, or badly written. That is all"?
The measure that this review is not to be trusted comes from the reference to "that squalid Ripley tale". Which one? There are five of them, of varying quality, but if all you have to say about any of them is "squalid", then your opinion can safely be disregarded.