"Immature sentimentality"

The NYT Book Review for July 17 carried a review by Randall Kennedy (prolific author on race relations and professor at Harvard Law School) of Harper Lee's Go Set a Watchman.  The 'teaser' for the review says (this is a close paraphrase): "Go Set a Watchman forces readers to abandon the immature sentimentality ingrained by middle school and the film version of To Kill a Mockingbird."  The nice thing about these teasers or summaries is that occasionally, as in this case, they make reading the review, if not exactly unnecessary, then perhaps optional.  One question though: Doesn't Atticus Finch lose the case in To Kill a Mockingbird? How "sentimental" can that outcome be? (Yes, I've seen the movie; no, I don't really remember it that well. Was a hell of a long time ago. And no, I haven't read the Kennedy review of Watchman yet.)