She means well, but she always manages to do the wrong thing. She has a real talent for it.
In the quote from Beverly Cleary's "Fifteen," the speaker acknowledges that the person they refer to has good intentions. Despite her good intentions, she consistently makes mistakes or causes unintended problems. Her efforts are genuine, but her actions often lead to the opposite of what she hopes to accomplish.
This highlights the idea that good intentions alone do not guarantee positive outcomes. It also suggests that her tendency to do the wrong thing might stem from a lack of awareness or experience, yet she still means well. The quote emphasizes the difference between intentions and results in people's actions.