The level of competition does not matter in the face of the law of large numbers. If two players each played 1,000 battles on the ladder, then the better of the two players would almost certainly achieve the better win-loss record regardless of how good or bad the players on the ladder are.

So consistently topping a dead ladder like AG or Ubers with an impressive GXE and W/L ratio is just as impressive as topping a stacked format like OU or VGC.