Is the theme of frailty in Atonement by Ian McEwan a major one?
Human frailty is definitely a major theme in Atonement. One could say it is the most important theme, because ultimately it was Briony's frailty and weakness of character that creates the overarching structure of the novel's story. Briony becomes a writer so that she may recreate reality, and give a more fair and happy ending to the romance of her sister Cecilia and Robbie. But she admits that the happy version of their story was one she made up, and she feels deep regret and sadness for her impact on their lives.
Briony's actions kept Robbie and Cecilia apart, because he went to prison after she implicated him in Lola's rape. To get out of prison, he offered to be drafted into the army, and he died on the battlefield. Her sister was killed working in London as a volunteer for the war effort, presumably in support of Robbie. The fates of her sister and her sister's lover might have been completely different if Briony's actions had been different, and her personal weaknesses (her selfishness, her dishonesty, her jealousy, and her emotional decision to take revenge on Robbie for rebuffing her childlike feelings for him) were responsible for these actions. The resulting guilt is Briony's lifelong motivation to atone for her actions.