People have been writing diaries for centuries.
It’s not like this is something particularly new; you wouldn’t be expected to have to ask permission from the other person in order to be able to write about them in your diary, I feel like that’s just a little excessive.
It would certainly feel weirder if someone had a journal specifically for journaling about me and cataloging facts about me, rather than me appearing in their narrative as a side character every now and then.
I also like to keep notes on my acquaintances. I think the “creepiness” level depends on why you are keeping them and the level of information you keep on them. As a regular rule of thumb, I write notes that if I have to show the note to the person, they would feel flattered, not, for lack of a better word, violated.
For instance, in one conversation with a friend, he/she casually says he/she likes to listen to old pop LP records. I make a note of that and on his/her birthday I can choose a present that I know he/she will like.
I wouldn’t make a note of a personal secret that the person told me.
So, I think it all depends on the motives of the note.