The tone and mood for this novel is actually very complex. On one hand, due to the nature of the story (the antihero HH is both a pedophile and a murderer and not all that stable), there is certainly a mood and tone of it being a cautionary tale and that it is morally wrong for HH to do what he did. He married a woman he was not in love with to be closer to her barely adolescent daughter and after she dies, he starts a relationship with Lolita. He eventually murders the man who took Lolita away from him. He then dies in prison, waiting for trial.
But at the same time, there is a rich, seductive tone to the novel itself. HH is a professor and the tone of his narration is also filled with intelligence, cleverness, and a rich, heady tone for his misdeeds. So the tone/mood to Lolita are directly contriditary which is part of why people continue to be so scandalized by it today, because on one hand, it's morally wrong and despicable, on the other, it's told in a sexy, smart way.