Mixed? I'm not sure what you're talking about.
But the guy above was talking about drinking single malt scotch on the rocks. It's not the temperature.
Single malt is enjoyed because of its taste and character, but if it gets mixed with enough water, from melting ice for example, it breaks and loses its appealing qualities.
Single Malt + Water = Former Delicious Whiskey. But in an emergency, you can use ice to make some awful blended whiskey potable.