I think Mayella would write of the relief she feels. By her testimony, Tom is sentenced, and that will appease her father. By the end of her testimony she realizes that everyone knows that her story isn't likely to be true, so her last, desparate attempt is to play "the race card." She essentially says, I am white, and you had better believe me. I think her fear of father's abuse overrides any other emotion she could have for anyone else.
Mayella Ewell would have written about her shame and humiliation, but I think she also would have written about her victory. Her behaviors were motivated by fear of her father, so she would have been victorious in deflecting his anger by testifying against Tom.
Firstly, it is unlikely that Mayella would write in a diary: she was probably illiterate. Despite this, I think she would be confused about her feelings towards Tom Robinson still. She ultimately stood by her father and his interpretation of events even though he offered her little in terms of respect, kindness and humanity - all of which she got from Tom in his brief but significant attempts to help her. Mayella would be aware of her own lowly status in the eyes of the law and the community and would be baffled by Atticus' eloquence and gentleness in questioning her. She would be used to rejection, maybe even abuse, from the rest of the white community and probably would not have understood him treating her with dignity despite the inaccuracy of her testimony. I think that Mayella would have liked her voice to be heard by the community not in a courtroom to condemn any man, but as an appeal her humanity as well as Tom's. She may be frustrated that she was unable to express herself as an individual but also awestruck that the power of her words could kill a man.
I would think Mayella would write about how embarrassed she was and how Atticus made her look stupid in front of the town. She would also talk about her father and his abusive nature towards her and how Tom was the only person who was nice to her -- and he wasn't "even human." (I say the "even human" part because many racists didn't look at African Americans as people with feelings so, even if she wasn't completely racist, her family and father were so she probably inherited their feelings towards the race.) She could also talk about her worthlessness as a person since she's stuck at home, being told how useless she is, and raising her family as well.