I really liked the first one, but something was missing from this one. One thing that is really important in movies is pacing. There needs to be proper climaxes and intense situations that cause you to be looking forward to each new scene. This movie just didn't have that element. The whole movie felt really humdrum. There wasn't a proper amount of time between the bad guy taking over, and the good guys rising up against him. They should have brought out the villain a little bit earlier, and given us a little more time to not like him. Then at the end, the bad guy kind of just disappeared. There was no satisfying end, death, or punishment for him besides just being on the dying dragon's back.
Most of all, the fact that Hiccup's mom had left him and his dad in order to help dragons is pretty messed up. I didn't like the mom because of this. The show wanted me to like the mom, but I didn't because she abandoned her family, and that is never ok, even to save the dragon species. All in all, it is probably worth a watch, especially if you liked the first one. But I don't think it lived up to expectations, or it's Rotten Tomatoes rating.