Yes, I'd say so. Although turkish food tends towards olive oil instead of butter, which would mean anything without meat and cheese is technically vegan, I've found that most restaurants don't consider it to be vegetarian / vegan. Basically, what that means for a vegan customer is that there's no guarantee there's no hidden meat sauce, butter, or re-used kitchen supplies (if that is a big deal for you). If you want vegan food at a restaurant, you'll have to be quite explicit- not just saying meatless (etsiz) but also being prepared to explain that you want no et suyu (meat sauces), süt (milk), or yumurta (egg). Turkish food is ABSOLUTELY worth this hassle, and the best two foods, patlıcan salatası (eggplant mash) and Baklava are vegan.
By the way, there's a vegetarian restaurant on the alley across from the Aya Triada church in Beyoğlu. It's called Govinda. There's also a great hummus and falafel place on the other side of Taksim Square called Falafel House.
Although veganism is not very common in Turkey, on the contrary to common belief, there are vegan restaurants or just normal restaurants offering vegan food pretty much in number. Here are some places that are loved and appreciated.
1) OSMANLI SARAY MUTFAĞI
Caferiye Sokak No 6/1, Sultanahmet
The kitchen is open between 12:00-24:00 every day and offers live Turkish Classical Music on Friday and Saturday evenings. You can have your breakfast between 7:00 and 10:30 and brunch is from 11:00 to 15:00 every Sunday. The menu includes recipes not only from Turkey but also from Middle Asia, the Balkans, Middle East, Mediterranean and Europe. As you can infer from the name of the place delicious Ottoman foods are offered here. This place is definetely worth seeing.
Büyükparmakkapı Street No:2 | Taksim, Beyoğlu
This little but really warm and cute place is open only between 12:00-23:00. It is closed on Sundays. You can find different choices in the menu for vegans and vegetarians for reasonable prices. If your accompany is not a vegan or vegetarian s/he can also has nice dishes including meat, chicken and fish.
3) FALLS IN GALATA
Büyük Hendek Street No:35 | Galata, Beyoğlu
This places is located in a really old building from 1800s. It has nice vegan foods, limited though. It offers all-you-can-eat buffet as breakfast. You should try "green mantı" :) If you would like to have a silent, relaxing and comfortable time around Galata, then this is the exact place for you.