Not really. I mean, look at the James Bond series. I mean, sure, there's lulls and bad spots in it, but it has always rebounded to deliver highly entertaining, engaging, and certainly not bland films.
Sequels generally have a bad reputation. Why? Because, a lot of the time, sequels are made of movies that have no need for a sequel - they're definite "one shots". With superhero movies, however, that's not a problem. You always have tons of comics to draw stories from.
So, I'm sure you ask, why do so many franchises go down in quality as the sequels go on? I honestly think it's a case of little more than bad luck...and lack of caring. While it may seem pretty easy to us, making a good movie is, well, really, really hard. And is really a rare thing. Of the hundreds of movies that come out in a year, probably only a dozen of them are worth a damn.
Thankfully, movies like Batman, and Spider-Man, and X-Men were all one of the worth a damn in their respective years. In large part, that's just luck. Who knew Raimi would fair SO well at making a Spider-Man movie? Or Burton with Batman? Or Singer with X-Men? They were - as most movies are - creative gambles. And they paid off. But, obvious, the more films that are released, the more chances there are for them to turn out bad, for them to lose their respective gambles. WB bet on Burton, and got a return, WB bet on Schumacher, and lost nearly everything.
Beyond that, it's also pretty obvious that, after a while, studios stop really caring about making a good movie, and just making a profit. If it's proven the Batman or Spider-Man name makes money, it seems as though the studio doesn't really care who's the director or writer; the character itself sells, who needs anything else? I really believe that's what happened with movies like Superman III and X-Men III.
And, lastly, sometimes good directors just make bad movies, or, at least, mediocre movies. Speilberg made A.I., Coppola made Dracula, Raimi made Spider-Man 3. The fact Raimi's film is a sequel really doesn't have that much to do with it, IMO.