It's funny because:
- it's a children's programme overdubbed with lots of (really bad) swears
- it is dubbed quite well
- it's Postman Pat! And nothing ever happened in Postman Pat!
- the idea of him secretly being a foul mouthed Geordie is just inherently funny.
It would have been funnier if it were shorter, or scripted better, but it
was funny imo.