Indeed they are:

Sharps Appalachian Diaries

The way I get to find Sharp's comments on a particular singer is to look up the date he collected songs from him/her, in the Roud Index, then go to those dates in the diary. Be aware that there is a full transcription on the site, as well the a facsimile of the (illegible) original.

The note books are online at The Full English.

You can search by collector, singer, song, place, date, etc., and there will be an image of the appropriate page in Sharp's notebook.   However, finding those additional notes on singers is complicated, since they are scrawled below random song transcriptions, like this:

Notes on Julie Boone

You need to go through a lot of images to find them - it's easier to use the paper copies at the Vaughan Williams Library and flick through until you find a song with some notes attached. I did make a few transcriptions when I was researching 'Sharp's Appalachian Harvest', which I could post when I have a bit more time, if anyone's interested.