Lots of movies with sequels are good. For example, Narnia had a great sequel, the second one may not have been good, but the first and third were great. Some sequels help give ground to the original story's plot. Sometimes it gives backgrounds that helps you understand the first one. Other times its just fun to keep the adventure going like Iron Man, Thor, Star Wars, etc.
Actually the sequel you are stating are not really sequel as it is extracted from the same comic book and they only had to divide the book into different parts to make more movies and more cash. A real sequel would be, jaws which as critics say was a total disaster with a poor visual achievement. Narnia like you say was good however that is your opinion, the main reason it was liked was because of humans natural need to be curious. Sequel most of the time largely go off track from the original idea and are only what a persons thought were, therefore anyone could have had imagined a different version, Percy Jackson was a total fail, with the first and second being out of context and the sequel worse and worse. Therefore sequel are bad in a way but yet good for human nature. But the real question is still here" are sequel really that good?" No as they differ from the original work and are only ones imagination to further the work wereas any individual has the capability to do just .
It is true that most movies/sequels are from books or comics, but more times than most the movie is just as good if not better than the book. Lots of people like the visual of the movie from the book. So the sequels like Harry Potter, Avengers, other Marvel movies, Divergent, Hunger Games, etc. are able to be visualized. Would you rather have just one movie of each, leaving cliffhangers and questions? No, people are fascinated and interested to see the next adventure, continue the action or mystery, even if it came from a comic or book. Yes, anyone can come up with a different ending, but that makes it better, if you don't like the ending provided, then you can create your own. Other times it is interesting and sometimes hilarious to see other people's perspective. Bad sequels are great examples to directors so they know how not to make the same mistake. Without bad sequels like as you say Jaws or in your opinion Percy Jackson, we would not have the knowledge how to make them better.