The Prius probably doesn't have a greater percentage of X bumper stickers than any other car, but because the Prius is distinctive looking, people tend to remember the times they saw it with a particular bumper sticker as opposed to the times that they saw some other car that just looks like every other car on the road with the same bumper sticker.
MPG goes down in any car as speed goes up. This is because the aerodynamic drag goes up as a square of the speed and the power required to overcome the aerodynamic resistance goes up with the cube of the speed. However, the .29 drag coefficient of the Prius makes the MPG reduction less than in cars with a higher drag coefficient. So you'll still get better MPG driving a Prius at 85 than driving almost anything else. You won't get better MPG driving at 85 than you would driving at 60.
In addition, driving at 85 mph is still in the inefficient power band for most cars which hurts MPG even more. The Prius' engine is sized to run in the efficient power band when crusing rather than when accelerating. Acceleration is taken care of by the motor/generators (MGs) which provide about 195 ft.lbs. of torque. This allows the Prius' engine to run at 34% to 38% efficiency rather than the 20% efficiency of old fashioned cars and still have plenty of acceleration power. Because the engine is an Atkinson cycle engine rather than an Otto cycle engine, pumping losses at part throttle are mostly eliminated. At very low power demands, where even the Prius' engine would be inefficient, the engine shuts off allowing the MGs to propel the car.