If Piper was 100% likeable, the show would be too perfect: an innocent white girl in prison, surrounded by gritty criminals. Boring. The show would probably fall victim to many of the TV sitcom stereotypes and would be predictable. Piper-The-Perfect-Princess would win over all the inmates in proper, Disney Princess fashion and rehabilitate them into perfect people, like her. There, I called it. (I wrote last week on the specific ones I hope the show avoids in its writing).
The portrayal of Piper as dislike-able makes the show feel a little more realistic and less campy.