If Balzac's goal was (as he claimed) to write a realist novel from his "own old pen" rather than mimic the style of Eugène Sue, history and literary criticism have declared him successful. William Stowe calls La Cousine Bette "a masterpiece of classical realism" and Bellos refers to it as "one of the great achievements of nineteenth-century realism", comparing it to War and Peace. Some sections of the book are criticized for being melodramatic, and Balzac biographer V. S. Pritchett even refers to a representative excerpt as "bad writing". Most critics, however, consider the moralistic elements of the novel deceptively complex, and some point out that the roman-feuilleton format required a certain level of titillation to keep readers engaged. Others indicate that Balzac's interest in the theatre was an important reason for the inclusion of melodramatic elements.
Balzac's trademark realism begins on the first page of the novel, wherein Crevel is described wearing a National Guard uniform, complete with the Légion d'honneur. Details from the 1830s also appear in the novel's geographic locations. The Hulot family home, for example, is found in the aristocratic area of Paris known as the Faubourg Saint-Germain. Bette's residence is on the opposite end of the social spectrum, in the impoverished residential area which surrounded the Louvre: "Les ténèbres, le silence, l'air glacial, la profondeur caverneuse du sol concourent à faire de ces maisons des espèces de cryptes, des tombeaux vivants." ("Darkness, silence, an icy chill, and the cavernous depth of the soil combine to make these houses a kind of crypt, tombs of the living.") Descriptions of her meager quarters are – as usual in Balzac's work – an acute reflection of her personality. The same is true of the Marneffe home at the outset: it contains "les trompeuses apparences de ce faux luxe" ("the illusory appearance of sham luxury"), from the shabby chairs in the drawing-room to the dust-coated bedroom.
Precise detail is not spared in descriptions of decay and disease, two vivid elements in the novel. Marneffe, for example, represents crapulence. His decrepit body is a symbol of society's weakness at the time, worn away from years of indulgence. The poison which kills Valérie and Crevel is also described in ghastly detail. The doctor Bianchon explains: "Ses dents et ses cheveux tombent, elle a l'aspect des lépreux, elle se fait horreur à elle-même; ses mains, épouvantables à voir, sont enflées et couvertes de pustules verdâtres; les ongles déchaussés restent dans les plaies qu'elle gratte; enfin, toutes les extrémités se détruisent dans la sanie qui les ronge." ("She is losing her hair and teeth, her skin is like a leper's, she is a horror to herself; her hands are horrible, covered with greenish pustules, her nails are loose, and the flesh is eaten away by the poisoned humors.")
La Cousine Bette is unapologetic in its bleak outlook, and makes blunt connections between characters' origins and behavior. For these reasons, it is considered a key antecedent to naturalist literature. Novelist Émile Zola called it an important "roman expérimental" ("experimental novel"), and praised its acute exploration of the characters' motivations. Some critics note that La Cousine Bette showed an evolution in Balzac's style – one which he had little time to develop. Pointing to the nuance of plot and comprehensive narration style, Stowe suggests that the novel "might in happier circumstances have marked the beginning of a new, mature 'late Balzac'".