Actually, it's not "campy" or "unintentionally funny."
Yes, Faye Dunaway is over-the-top as Joan Crawford, but Joan Crawford herself was over-the-top in real life. Many people forget that.
People laugh at it because it has scenes that come off as being "full of Narm" when interpreted out of context. In context, they're terrifying and well-performed. And anyone who laughs at this movie or finds it "campy" must love child abuse (because there's a lot of that in it) and needs therapy.
Also, there's a scene that people laugh at which actually happened - Joan Crawford's daughter becoming sick and Joan herself replacing her on an episode of a certain show she was on.
Go watch the Cinema Snob's review of the movie. He said in his review that the movie didn't deserve the hate it got (especially from the Golden Raspberry Awards, which don't even see or hear anything they nominate - like all entertainment awards shows - and they nominated Stanley Kubrick a "Worst Director" Golden Raspberry Award for The Shining. Nominating him was one of their biggest mistakes, just like CRASH winning "Best Picture" is one of the Oscars' biggest mistakes and CRASH being rated "Fresh" on the Tomato Meter is one of Rotten Tomatoes' biggest mistakes, since RT sometimes rates bad movies (such as Tim Burton's Charlie and the Chocolate Factory and Alice in Wonderland 2010) "Fresh" and good movies (such as Mommie Dearest and the 90's Teenage Mutant Ninja Turtles movie) "Rotten") and that Faye Dunaway gives one of the greatest performances in cinematic history. Period.