Contractual Purity: Averted in her career but not in public consciousness as many people don't know about her post-Hollywood accomplishments. Not to mention that she was a smoker in her later years, hiding it from the public as that wouldn't sit well with her younger fans.
Tastes Like Diabetes: A modern teenage girl (a child of the early 2000s) was quoted online as saying that sap must drip from the film reels.
A lesser-known fact is that her manager-mother didn't like the sugary goop either, and wanted Shirley cast in more peppery, even bratty roles. The image she had in mind was Mary Pickford, at best kind of a Plucky Girl / Heartwarming Orphan type. Shirley wanted to play Action Girl type adventurers. Neither of them got much of what they wanted.
Values Dissonance: Her films worked in the 1930s, when such unashamed cheerfulness was needed, but now... not so much.
It also didn't help that some would view her interactions with adults in her films, which while not necessarily malicious, as being creepy or borderline paedophilic. The "Good Ship Lollipop" musical performance in Bright Eyes for one gained notoriety in recent years due to Shirley being alongside a band of grown men inside a passenger plane. It can be attributed to the moral panic caused by child predators and fears of minors being molested or taken advantage of, though others criticised the negative reception to the scene as being overly cynical and dirty-minded, as well as furthering the stigma associated with men and children.
She got married at 17 to a "war hero" in order to get out from under her parents' control. Her new husband ran through her money, beat her black and blue, and used her connections to get work in Hollywood, trashing her good name so she wouldn't be able to find work and would have to be dependent on him. When she divorced this Jerkass, she was universally condemned for "breaking up her marriage".