It's of it's time. Started as a weekly paper column thing.
Then the books.
As pp said diaries are internal life, feelings etc. Having a rant to get things out system.
They tracked her constant efforts to drink less, lose weight in that sort of I ought to but well never stick to it for more than a few days because friends work life enjoy having fun. Which I'm sure lots of women in general can easily relate to.
Her seeing other women at work and thinking they're no much more elegant, attractive, together than me.
Her thoughts about colleagues esp when had bad day and sometimes they turned out to be nice really and sometimes not.
Her hilarious embarrassing stuff was from her cringing POV and we always think things were much worse than others with that stuff.
Written it was v funny. And clearly she was attractive, successful, had plenty friends etc. It was how she saw herself. The whole point was that it wasn't accurate documentary writing. It was about insecurities, confidence, secret worries.
As pp said that really doesn't translate well to film.
Well except peep show comedy genius 
The films are of their time of course as so much stuff is. They have dated a lot. Not just these films!
I think if never seen then wtf yes.
If remember from time and enjoyed then way more likely to enjoy now even if being yikes sometimes!
I mean people still watch carry on camping etc... And over Xmas seasonal channel had Benny hill on FFS!