Both are used though "of" is more
common.
You can't simply compare usage like this, unfortunately.
Different prepositions have different meanings, and Google Ngram viewer doesn't understand meaning.
This is what I think 99 bottles probably wants to say:
I'm the prime target of the school bully.
Very generally speaking, one of the main uses of the preposition
for is to express a sense of
purpose. When I say
This coffee is for you, I mean to say that you are the intended recipient of the coffee. If I were to say
I'm a target for the bully, it would mean that I'm the intended target for the purposes of bullying.
Is 'of' used when talking about a specific person and 'for' when talking more generally?
No.