I think the movie is very funny.

And also very sad.

Great comedy is often sad.

This is what Solondz had to say:
“It’s often hard for me to separate what I find so sad from what I find so funny. There’s a kind of poignancy for me…things that I am very moved by I find funny.”

The genius of the movie is that the characters often act in a ridiculous way and that ridiculous things happen to them, but they don't see it that way. That's the comedy.

For example: Joy breaks up with Andrew.

At first there is a misdirect. It seems like Andrew will react with quiet resignation.

But then he gets extremely angry. Which is sad, but also funny.

It's funny because Andrew doesn't understand that the gift he bought for Joy isn't romantic or considerate so taking back the gift is a ridiculous gesture.

It's even more funny because Joy pretends the gift is great, because she thinks Andrew is being considerate and understanding.

Then Andrew goes off and starts ranting.

The irony is that the sympathy we felt for him wasn't earned. Joy was right to break up with him, but she broke up with him for the wrong reasons.

The whole scene is set up like a classic joke.

There is a setup, a misdirect, our expectation is subverted, and there is a punchline.

The scene would be far less funny if the gift had been an expensive bracelet instead of an ashtray.

Or if Andrew had been good looking. Or if Joy had been less oblivious.

The scene were the pedophile is trying to rape a child is extremely disturbing, but also funny.

It's funny (and extremely uncomfortable) because we are forced to see things from the perspective of a child molester and there is a gap between how mundane his initial actions are, and the terrible reality of what he's trying to do.

Plus there is the irony that this man is a therapist.

The scene with the son isn't funny, and there is a specific reason for that: it's a scene in which the therapist has to deal with how depraved he is. he has to see himself for what he is, a monster.