Man, a movie can't tarnish prior movies unless it introduces something that changes the context previous events and characters must be viewed through, something which does not apply here since these movies are unrelated aside from having a common writer-director. The Happening is also not that bad of a movie if you go in expecting it to be sort of dumb and how could you not expect it to be dumb when it has such an awful title?
One has to remember the intense hype that sprung up around Mr. Shyamalaman after Sixth Sense hit. He was touted as a rising star auteur, a master of taut thrillers in the vein of Spielberg at his best (people specifically noted Jaws and Duel, both of which hold up today) Then came another movie with third-act plot twist. Then another. Then another. By the time Lady in the Water came out Shimmelman was only really being defended as a good director and not a good writer. That film put Schoolmarm's rather massive ego on display, including himself in the narrative and including characters obviously modeled after his critics solely for the purposes of dooming them to violent ends.
Then The Happening came out, and I disagree with your assertion that it's not that bad of a movie. It's that bad. It's worse. It is transcendentally bad. And to make matters worse, beyond the curiously uninspired direction it was very recognizable as a Shambliss film. Except here all the things that were perceived as strengths were actually weaknesses. The tension-building was ridiculous, as the threat to the characters was ridiculous. More importantly, Shasta McNasty's terse and economical writing, once seen as an asset, was super fucking dumb ("There appears to be an event happening!"; "We've lost contact" "With whom?!" "(pause) With everyone!") He changed his PR course rather quickly after the movie's release, claiming it was always meant to be awful, but I don't believe him.
And ultimately that
does reflect poorly on his prior work, because there's not much of a good reason to say that two movies with several common central elements and a shared visionary can be vastly different. You watch The Happening and recognize that the writing is the stuff of sucky urban legend. You watch the Sixth sense and it has a very similar writing style that is considered a strength in one film and a crippling injury in the other, and you begin to wonder if the other's acclaim was mainly a boon of circumstance, the first cut being the deepest and all that. To be sure The Sixth Sense
is a greater film than The Happening. But could it be the case not that Shimmerlong didn't start strong and got weaker, as the conventional wisdom goes, but rather that he was always mediocre and simply spent all his capital over time and ultimately failed? How would we view The Happening differently had it come out before people were tired of Shamwow's schtick? Before they even knew who he was? What if Sixth Sense had been last? Wider context often informs the way we view things we enjoy - knowing the stories behind the making of "Rumours" or the Pop/Bowie/Reed Berlin Recordings enhances the experience of listening to them. Reading the story of the Hunchback of Notre Dame makes watching the Disney version ridiculous and perhaps a little unsettling. Lots of people saw The Sixth Sense without knowing what a skilled hack the man behind it was. There are elements of film that are objective, but when we're talking about artists and their patterns, and when passing gestalt judgments, perspective is rather important. It's partly a philosophical dilemma - If I like a band's single and decide it's their best track only to discover that they have dozens of nearly-identical songs, couldn't I have possibly heard one of their other songs and decided
it was the best? How can I be sure of my judgment?
I'm reminded of a story I read a long time ago. A writer at Slate was lavishing praise on the Aphex Twin for releasing a remix of Nine Inch Nails that the writer thought was a perfect parody of industrial music - It was all steam whistles and sheet metal percussion and minor-key, movie villain horns. To the writer, the remix was an example of Aphex Twin's talent for recognizing ridiculousness in music and coaxing the good out of it. However I knew something that the writer didn't - Aphex Twin didn't really intentionally make the remix. He was paid for it in advance and forgot about it until the deadline rolled around and the record company started calling. Not wanting to put such effort into something on a small time frame, he picked a tape off his shelf of unfinished / unreleased works, added some samples of the Nine Inch Nails song at the end, and sent it off as an official remix. That it came out the way it did was largely luck. Having known that, the awesomeness of the track (and I think it is very good) was lessened a great deal, because one wonders if the Aphex Twin puts a similar lack of effort into everything he does and if so, how much better his work would be if he applied himself. But the author didn't know that, and so the work was evidence of genius that was in reality just providence. Which one of us is right?