Great posts, especially those mentioning the geraniums, the other children, and the father.
We learn during the trial that Mayella can read and write but has had only two or three years of schooling, so the writing in her diary would not be poiished at all. However, we may also see her as intelligent. As I understand her interactions with Atticus during the trial, she is not stupid or confused; she repeatedly asks for clarification of his questions because she knows that words count a lot in the courtroom, and she doesn't want to give away anything that would weaken her case or reveal the lies and the truths behind her account of what happened.
I think it's interesting to think about how the diary entries would look in addition to what those entries would say. The handwriting might look child-like, and the content might suggest more than it says directly. For example, there might very well be repeated references to Tom Robinson passing by the house, day after day -- some brief, some describing in detail an odd job that he does for her -- but no full description of her full plot to lure him in.