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.
The use of "of" makes sense when you consider it as a possessive. If you change the sentence to "I am the school bully's prime target", it's clearly a possessive so "prime target of the school bully" is the clear way to reword it.