Rapunzel spent most of her life locked in a tower, getting manipulated, deceived and gaslit by a surrogate mother figure who kidnapped her as a baby. Despite this, she retained her kind-hearted nature, being optimistic, cheerful and always ready to see the best in people and give them second chances. This combination makes her extremely vulnerable to hypnosis, deception and brainwashing. She's not stupid by any means (far from it-, but far too trusting and is perhaps slow to recognise ill intent in others.

As >>8908887 has said, she gets into so many hypnotic or mental situations in her animated series it makes you wonder if one of the writers had a thing for it, or if it was merely a convenient reusable plot device to explore her character. Her brief encounter with Kaa in the Disney anniversary trailer adds more fuel to this fire of speculation. It could just be that they wanted the girl with the cast-iron frying pan to whack the snake, but for those "in the know" it' very interesting. She's demonstrated that she's extremely weak to his powerset, so she'd simply have succumbed to his charms the instant he laid eyes on her. An alternate take to explain her confidence in this short scene is that she's aware of what he can do and knows to strike hard and fast before he can hypnotise her. However, this implies she has hard-won knowledge of his abilities, perhaps after having been hypnotised a few times before. Either way, it's fun to speculate/fantasise about Rapunzel and Kaa having had multiple encounters.

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