Usually always the case. The next movies are just riding on the success of the first. I haven't watched Iron Man 3 but I still liked Iron Man 2.
This is a really hard question for me, because I grew up reading all of the Marvel comic books and liked just about all of them. I also read all of the DC comic books, way back when. So, I may be remembering how much I liked the comic book characters when seeing them on the big screen. That said, I think that I have to go with the Fantastic Four.
I think that the first movie has a lot of leeway as to what they can do and how they develop the story. Once it has become super successful, the money people insist that the sequel(s) follow a formulaic approach based upon what they think worked in the first film to try to ensure that they achieve the same financial results as the first. This tends to limit the creativity of the artists involved and leads to making a "caricature" of the first film. This is not true all for all sequels, but happens a lot, in my opinion.
If you're of the right age, temperament and sense of humor, Guardians of the Galaxy is pretty much the best of the lot. The sharp and witty dialogue holds up to repeated viewings, the plot and pacing is terrific, and the soundtrack is awesome. Peter Quill is like this generation's Hans Solo. To be honest, I am actually more looking forward to Guardians of the Galaxy 2 compared to Avengers: Age of Ultron. But deep down, I know part two won't be as well written.
Probably a combination of both. Expectations too high and they don't put as much effort into the sequels. I think a rare exception to this was Dawn of the Planet of the Apes. Thought this movie was great and is better then Rise of the Planet of the Apes.