I think there are a couple of ways to look at what he said.
The first is that he is completely correct about some of the fan subculture. Every single viewer of the show had nearly a decade worth of investment in the finale, which is obviously a massive expectation. Everyone had their own ideas for how it should wrap up, and when what they wanted(or what they read on some subreddit) didn't happen, they got mad. The same sort of subculture partially hurt the Star Wars sequel trilogy, and it effects plenty of other franchises as well. Everyone thinks that their idea or the one that they like is the best, and that is only natural. Personally, I didn't love how Thrones wrapped up. However, I also think it was in keeping with the general vibe of the show. Ramsay Bolton has a line in the middle of the series where he says "If you think this has a happy ending, you haven't been paying attention". So when the best people of the show don't win, it should have been expected. The theory was right, but their execution was lacking.
The second is that Dinklage doesn't seem to realize that people HAVE moved on from Game of Thrones. Not just from the finale, but from the show on a whole. The show literally permeated every single social sphere online and basically every single conversation right up until the point it ended. Then, it disappeared entirely. I honestly can't think of another show that had that same sort of drop-off as far as relevance. That is a HUGE testament to how bummed out people were with the ending.
I'm sad for the fate of Thrones, but I'm happy that we are getting another shot at the world with the prequel series