IMO yes it was severely overrated. To me this is the type of movie that many people feel like they should love out of pure moralism as it has both ethnic and cultural frictions as environmental/climate issues as underlying major themes, which are very relevant in our current society, and that you are obliged to praise this movie because of that. It's like the equivalent of liking things of FB. You don't actually do much, but you feel like you did something worthwhile rather than actively trying to change these problems.
You also then have that group of people that are all like "awh, cripple guy in an artificial body falls in love with alien princess, how romantic". Bleh. The movie often gets described as Pocahantas in space and that's exactly what it is.
In the end it's a subjective opinion and overall seen it can be described as a decent movie. But seriously it really doesn't deserve to be the highest grossing movie of all time. It was one of the few movies where I actually was rooting for the bad guys as the moralistic value of it was making me nauseous.