It is clearly possible to take both of these views. In one sense, Nicola exists less as an independent character and more as a carefully constructed male fantasy, embodying mysterious desire and erotic sensibility. Note how Nicola is described early on in the novel:
Her mouth was full, and unusually wide. Her mother had always said it was a whore's mouth. It seemed to have an extra half-inch on either wing, like the mouth of a clowngirl in pornography... Nicola's face was always dark, and her teeth had a shadowy lustre, slanting inwards, as if to balance the breadth of the lips, or just the the suction of the devouring soul.
In one sense, therefore, Nicola, with her sexual activity and her appearance seems to be more a product of a male fantasy than a real independent character in her own right. She deliberately exploits and plays on these fantasies with both Keith and Guy, and, to a certain extent, Samson Young himself. Remember the reader is only presented Nicola through the words of Samson Young, who, it must be noted, is not the most reliable of narrators and is himself male.
However, at the same time Nicola is presented as a very complex and manipulative character. She deliberately deceives all three of the male characters in the novel, including the narrator himself, Samson Young, and is presented in many ways as an enigma who constantly baffles the efforts of everyone to discover her true identity. As a complete narcissist, she is obsessed with the search for somebody to kill her and looks forward eagerly to her "appointed deathday" whilst at the same time caring little for those around her, only seeing them as tools for her use. Thus it is possible to view her as a deeply complex and fully fleshed character, who is the extreme product of a narcissistic, navel-gazing society that has lost all sense of meaning in a materialistic world.