To determine whether or not Montesquieu's Persian Letters is an Orientalist text, we must begin by examining the nature of Orientalism. According to Edward W. Said's 1978 book Orientalism, the concept refers to a skewed presentation of the peoples and customs of the Middle East and Asia. Some Westerners used such stereotypes to discredit and denigrate Asians and Middle Easterners, trying to show them as inferior and thus in need of western influence and control. Many images of Orientalism have found their way into popular culture, including exotic harems, snake charmers, and sheiks on camels. Not all artists and writers who use such images intend them as insulting; they have simply become standard representations.
Montesquieu's Persian Letters does indeed present its Persian main characters in stereotypical ways, at least in part. Usbek has a harem with five wives who are guarded and managed by eunuchs. These wives are quite lascivious, jealous, and temperamental, and those in charge of them are not especially adept at handling them. By the end of the book, there is an uprising in the seraglio that leads to chaos, murder, and suicide. We can see the Orientalist flavor here with the stereotypical images of harems, lust, and uncontrolled violence.
Of course, Montesquieu does not necessarily intend for those images to dominate his book. Rather, he uses his two Persian characters to comment (often quite satirically) on the customs and peoples of western, especially French, society and culture. He is trying to put some distance between himself and his environment through these two Persians so that he can look at the West and all its foibles in a new way and criticize them thoroughly.
Persian Letters, then, is an Orientalist text although not completely and not necessarily by design. Montesquieu likely used the images and ideas available to him in the wider culture, applying them to his particular purposes without recognizing them as stereotypical.