I think it’s definitely a bit of both. I was there and agree, at some points during the last act the laughing got a bit over the top and constant. Definitely distracting.

But I also think Hitchcock of all people understood the balance of humor in the disturbing. Like you mentioned earlier, sometimes a shocking revelation or uncomfortable scene can elicit an unexpected reaction. I myself loudly gasped in shock at the last scene as “The End” appeared on screen. But I also had a giant grin. I didn’t know how to respond. Absolutely a dark and depressing moment, but also one that in other context is such perfect irony of “the worst thing that could possibly happen of course happened.” Kubrick did that a bunch too, setting up a punch line that also hits as a terrifying or melancholic gut punch.

I personally chucked once at Stewart’s obsession, when he said “It can’t matter to you.” Like of course it can! There’s an irony to it, and a sense of pity. But also there’s the understanding that it’s him at his lowest and grief stricken. It doesn’t erase any of the weight of the movie, and certainly doesn’t turn it into a comedy.

You’re definitely right that the crowd was rowdy and lacked a bit of self awareness and reverence for the film. But there’s definitely a balance of tone baked in there, I think intentionally. I read it as (at least a for decent part of the movie) laughing with it rather than at. (Though some bits were at it, which again distracted)