Not really! I don't really know how to explain this, but.. Let me tell you why /meydanı/ is not capitalized. When you say Taksim, people only think of the the square. If there would be a another meaning of it, like there would be two things called Taksim, then yes, the square, /meydanı/ should've capitalized. But like I said, Taksim meydanı is right.