Is an ekranoplan less efficient than a regular plane?
There are a couple of inaccurate trains of thought in the question.


  The ekranoplan uses more thrust to fly in ground effect because it has to overcome air drag which is significantly stronger due to high angle of attack required to maintain the ground effect.


All aeroplanes need to overcome air drag, which grows quadratically with airspeed. Ground effect creates more lift at a given speed, therefore requires less thrust at that speed. Or: ground effect creates more lift at a given wing area, requiring less thrust than the larger wing required for the conventional aeroplane.


  ...thicken the air cushion to the point where the ground effect is lost and the aircraft will loose altitude down to ground effect.


Thickening an air cushion cannot result in losing ground effect. The ekranoplane cannot fly out of ground effect - increasing Angle of Attack does not make much sense and altitude will auto stabilise to within ground effect, indeed at higher thrust level at higher AoA for a given speed. I believe this is what OP means, but it does not equate to ekranoplanes being less efficient than conventional aeroplanes. Ground effect creates extra lift without a drag penalty.

Whether ekranoplanes are more or less fuel efficient than standard aeroplanes is not due to the ground effect, but to the interference with the water surface, sea state, having to lift the engine out of the spray etc. Depending on the speed, they may be less efficient than ships, but the payload arrives a lot quicker.