A "good diary" will show your development as an artist throughout your HSC year.
It does not matter how many ideas you had, or whether those ideas were carried through to your major work. It's to show a process and development of ideas.
As lislabella mentioned, it's only ever going to be an issue if you get over 43 (or whatever the Art Express cutoff is) or if markers believe you've plagiarised. Ask your teacher whether or not she thinks you have a chance for art express. If so, put some effort into padding out your diary. Or if your work is built around appropriation i'd probably make sure it'd well doumented, just in case.
Personally, I don't think I have much of a chance, so my diary is pretty patchy. I've got it about 4/5s full of random stuff and ideas, and the rest with my idea that went through to my final pieces. I'll probably put some more stuff in the diary in spare art classes between now and the end of school, as our teacher usually likes to make an exhibition of hsc works for students, which includes diaries.
Probably wasn't because of your diary. I know of a few people who were in the selection pool for art express but were turned away. And I know one of them had the greatest diaries i'd ever seen. About four A3 diaries FULL of brilliant works and progress.
From what various art teachers tell me, the selectors look for experimental/newish works. Mind you, i'm not sure how valid their comments have been. Considering when I went to artexpress last year I saw a lot of crap, that lacked innovation and originality.
