That’s a good point. It’s as if Spielberg, Lucas and company were thinking, “Right, after going so dark with TOD, we need to get back to the “fun” of Raiders! Everyone always says how “fun” Raiders is, right? Let’s make the next one fun again!”
But they missed the point of what made Raiders “fun”. It was the thrills and dangers, and the clever ways Indy got out of trouble. It wasn’t “fun” because it was lighthearted and funny. It was fun because it was a thrilling edge of your seat adventure with real danger that the characters just barely escape. The small bits of humor work in the previous movies because they’re there to balance off the real perils and close calls. They’re a great release after an intense scene that winds up the audience.
Of course, I actually wouldn’t mind a “funny” Indy movie if it was actually, you know, funny. Take Star Trek 4 for example. That was a real tonal shift from the previous movies, and pretty risky to take a beloved franchise and make a comedy. But it worked because the characters were true to themselves and the humor arose from the situations they were in. And the Marvel movies are pretty funny too (although their quality is highly debatable around here!). So it’s possible to make a good comedy action movie. But it’s damn hard and you better be sure you get it right.
Getting back to the shoddy effects in TLC, I do have to point out that TOD also has a couple of bad scenes. The group riding the raft down the mountain with the rear projection looks incredibly cheap. And wide shot of the group on the side of the cliff with the water bursting out also looks pretty bad, and did back then too.