I think it's about personal experience. Are you someone who likes to turn the AC on high and keep your house/place of work cool during the summer? You may find it a bit hot. They do tend to keep the cabins in the mid 70's fahrenheit. Air condition isn't as common in Turkey, so they set it to a temperature most of their passengers are comfortable at.

That said, personally last summer flying in J JFK-IST, I boarded wearing a polo shirt and khaki pants and switched into shorts and a t-shirt as the flight attendants prepared the bed. Was never too warm with the blanket half on, half off.