I really do believe the sequels will do well overseas and cannot see them as being international flops. Might they flop domestically? Yes, but who cares if your movie takes in over 1 billion worldwide?
Also, as Sheikra pointed out, people may think the movie is crap, but they still agree that it looked pretty. As far as building a whole new themed land goes, that is one of the biggest selling points. Very few guests want to walk into a gross looking land (unless they are visiting HHN) so having a nice looking area in the park will do well. In all reality, the 'story' of the rides matters about as much as the 'story' of the movies. It is all just pretty to look at and will be attractive to guests.
I really do not think how much money the movie rakes in (or does not rake in) is going to matter at all to how popular the land ends up being.
Edit: Given that The Magic Kingdom is the real draw to the Disney parks with the other three serving as just support (or add ons to your ticket) I really doubt that AvatarLand can fail.