I'm not a techie but I'll have a go and happy to be corrected.
As I understand it. ESP monitors various things the car is up to: braking, acceleration, steering wheel angle, accelerator pedal depression. If there is something suden on any of these, because you brake or swerve fiercely or lose grip on a driven wheel, ESP intervenes to restore the situation to "normal".
If I'm right it will only operate when it has to and so I don't think there will be any overall effect on mpg. I think systems can be turned off so the car won't intervene when you're deliberately trying to go side-ways at 85 mph.