Sibel Kekilli is not a "porn star". Sure, she acted in a few porn movies at the beginning of her career, but nobody knew her at the time and it's not until she became well-known for a great performance in a movie that piled up the European awards (Head-On) that the embarrassing truth about her former career became more widely known. And what does her accent have to do with it? Shae is clearly a foreigner with an accent in the books as well, as I recall.

Though that said, I don't necessarily disagree with your assessment of her role in the series (having only seen the first two seasons yet). She seems to lack what she does have in the books - a keen awareness of the eternally precarious position of a king's or nobleman's mistress, who may exert a great influence upon him but always has to remember that she has no formal power or even equality of rank, and can be unceremoniously dumped into the gutter at a moment's notice.

That other great European actress wasted on a small role (Carice van Houten, as Melisandre) does a better job with her similar position - or the writers did a better job, more like.