I'm a little surprised by the amount of criticism of Hapu. People have been stealing her radishes, so of course she would be a little on edge. She caught Ash climbing over the fence into her field. Her assuming he was the radish thief was perfectly understandable. I would probably draw the same conclusion if I were in her shoes.
Also keep in mind what Nurse Joy said about her. She was the only person her age around, so she interacted with—and battled against—only adults. It's not hard to imagine the adults, being adults, patronized her because she is a child, which would only harden her personality. I saw nothing foolish about her character in this episode. She was a farmer defending her livelihood, and she apologized to Ash when she realized she was in the wrong. On top of that, she is the granddaughter of a former Kahuna, so she had top-notch battle instruction. If anything, her back story, as brief as it was, explained a little bit more about her personality.
Overall, I liked this episode. It was a good introduction to Poni Island and nicely set up this island's "arcs" for the other characters. Ash's battle against Hapu cemented her as a tough opponent. Her battle against Team Skull would have been more impressive if it had been against anyone else, but it was a nice show of Musdale's Stamina Ability.