You would have to work hard to get a bad meal in Istanbul. I would suggest trying for categories of restaurants or foods: lokantasi (often cafeteria-style, easy for non Turkish speakers because you can point at the food you want!), the fancier fish places on the shores of the Bosphorus, a place with outdoor dining in a restaurant-heavy neighborhood like Beyoglu or Kadikoy, a kebab place, a borek place, etc. I spent a LOT of time in Istanbul during my career but can barely recall names of places, because most of the time I was either brought to them by Turkish colleagues or just wandered in off the street with no plan.
There are several very good food blogs about Istanbul, some done by Turkish people and some by expats. People in Istanbul LOVE eating out and they can steer you in a good direction.
