It is campy and light but that isn't all it can be. Their villains and stories can be dark. Doom should be a villain the audience loves to hate. Someone who they are terrified of. He was the character who inspired Darth Vader he should darn well act like it! Read the Unthinkable arc by Mark Waid and Ringo. Claremont's run had some dark moments, as well. The comedy certainly shouldn't be the primary focus like what Fox did.
Still unacceptable for an FF movie, IMO.
Your pigeonholing a genre here. Not all super-heroes are "fun". They can range from campy to hilarious to crazy to depressing. It just depends on what franchise is being adapted.
Super-heroes have different sub-genres, too. WW is fantasy, GL is space opera, Superman is sci-fi, IM is hard sci-fi, Ghost Rider is horror, DD is hard boiled crime fiction. They can also switch sub-genres once in a while. Some get short story lines different to the status quo just to shake things up for an arc/creator run or two which are magic based, mysteries, intruige, funny stories, horror etc.
Some super-hero franchises can be lighter fare then others.