Miss Strangeworth spends her energies in tearing down others. She believes herself to be of the highest character, almost a type of nobility for their small town. Thus, she also believes that she is justified in passing along hurtful information, whether the information itself is true or not. She believes that evil must be rooted out, and by keeping everyone suspicious and on guard, she facilitates this effort.
Instead of confronting people directly, Miss Strangeworth works in secret. She doesn't write her letters on the stationery bearing her name and never signs her letters, either. She walks to the post office alone and includes no return address. In every way, Miss Strangeworth attempts to make certain that her identity cannot be discovered as she ruins lives around town. Sending out these letters is an act that she knows is "harsh," but she believes that exposing the wickedness of the town is worth it.
I'd say the punishment is appropriate for a couple of reasons. First, the actions are done in private, just as Miss Strangeworth herself operates. Instead of confronting Miss Strangeworth about the accusations in the letter, the writer of this letter works behind the scenes, mimicking the way Miss Strangeworth has always handled potential conflict. It's also appropriate because the writer strikes at Miss Strangeworth through something that she holds precious, just as she attacks lives in town by attacking things which citizens find precious, such as their marriages and children.
The writer of the letter most likely hoped to cause a character change in Miss Strangeworth by inflicting some pain on her, helping her to understand the way she has been hurting others. Unfortunately, destroying her roses seems to have only confirmed Miss Strangeworth's beliefs that she lives in a wicked world and that she herself stands apart from it. Therefore, the actions taken against her have not been appropriate for character change.