I think it's the bitter tone of the episode, with Monica being mad at Rachel. It was the night of their proposal, it should have been funner. To add to it, the reason that Rachel gave Monica for kissing Ross was kinda weak: because she was sad. There are some funny moments.
Like the beginning where Monica yelled in the balcony "I'm engaged" and Ross thinks it was a kid shouting "I'm gay"...and then it was explained that the kid moved.
I liked a lot of Phoebe's scenes trying to get Monica and Chandler to let her sing at the wedding. And Phoebe warns Monica, "Next time you touch my guitar, I'm gonna have to pound on you a litte."
The Joey having to audition as a 19-year-old was enjoyable, if only for the sight gag of seeing Joey's...I mean Chandler's boxers. The Chandler having problems in the bedroom was kinda okay if only for the line "We've take a nice walk on the Rue de la blakdflasdjf"...which is a joke I stole in one of my stories.
Speaking of Chadler, Matthew Perry's pain-killer drug problem could be seen again, as he is skinny, almost as anorexic as he was in season 3.
There's always a reason I look forward to if I see an episode, and that reason was Ross and his hands joke...and also his "hmph" as he walks to Rachel's room hoping he could get sex.
So overall, it's not that the jokes that was bad, it was just the tone and mood which didn't feel right for the episode.