QuoteName me another 'monster' design that was awarded an Academy Award.
King Kong (1976 and 2005), ET, Jurassic Park, Terminator 2.
Movies don't win for 'creature design' alone. Nicky Allder and Brian Johnson had nothing to do with the creation of the creature, but still got Oscars.
And I'm not seeing how the Predator is much more complex either. It's 'the most dangerous game' with an alien instead of a rich dude. Not terribly deep. In fact, probably less deep than the source material. Aliens have a habit of hunting and killing humans in movies. Humans don't usually hunt other humans for sport.
The Predator, due to it's human-like qualities, may have more scope to become actual characters rather than simply monsters, but I don't think it's really been on display thus far that much.