Spielberg's movies tend to lurch between "OMG BEST MOVIE EVER"-good and "...how does he still have a job?"-bad.
Temple of Doom is a good example. George Lucas wrote the story to be something really, really dark. Something that delved deep into the blackened, charred heart of an ancient and vile cult. None of the other Indiana Jones movies quite had the same imagery in it - other people died in horrible ways (face melting), but it was always an accident. Cursed by their own hubris and such.
In the Temple of Doom we see people being ritualistically executed.
So how did Steven Spielberg choose to convey this sense of dread? By making it the most light-hearted, colorful movie in the entire series. Adding two lovably clumsy sidekicks certainly added to the urgency and sense of peril!
George Lucas is good with stories. George Lucas is, unfortunately, bad behind a camera. I think that's what made the Raiders of the Lost Ark and The Last Cruade so powerful - Steven Spielberg's directoral skill paired with George Lucas' storytelling.
I'm willing to give Lucas the benefit of the doubt here. Considering the fact that, usually, Lucas is the one with giant dollar signs in his eyes, there must have been something horribly... unprofitable... in the script.