in regard to her diary, Cecily tells Algernon, "You see, it is simply a very young girl's record of her own thoughts and impressions, and consequently meant for publication." (act 2) At whom is this aimed?

The statement is just teasing, flirtation between Cecily and Algernon. She had an earlier conversation where she spoke about keeping memories with Miss Prism. Miss Prism said that memories are what you carry around with you, the things you store in your brain; Cecily's response was that novel's were created from stored memories that were only written down the way the person remembered them, and that they were generally inaccurate. Diaries on the other hand were meant for accuracy.