Thing is if you read the FFO in order Ern is not "Ern Goon, the local bobby's nice, but working-class nephew, who is presented as comic relief because of his accent, vocabulary, morals and manners..."
In the first book, he's pushed on them by a mixture of Goon and himself. Because he's come, Goon gets two of the parents to agree that they won't solve mysteries so that they don't get Ern into trouble.
Ern follows them round and repeats back all they say to Goon. He's the one who is turning up to join in uninvited, has got them into trouble with their parents etc. How many groups of children would actually have put up with that?
They're a close knit group who don't really want others to join them - much as the original four are in the first story when they don't really want Fatty to join them. They put up with Fatty at the beginning because they like his dog, and they're not especially nice to him. Gradually they come to accept him but it's only in the third book that he becomes their leader, a little grudgingly, but they admit he is the best at mysteries.
In the same way Ern goes from being a bit of a clingy pain through to someone they trust and are genuinely pleased to see.
The teasing of his vocabulary is mostly because he has a habit of running words together "'SwatIsaid" and that when he's writing his "pomes" he uses very flowery language. Only manners one I can think is where he's forgotten to take his hat off inside and Pip and Bet's Mum, who is very strict, reprimands him - and she reprimands various FFO-ters for similar infractions.
By the end of the series Ern is very much regarded as one of them and treated by the children as such.
It's very clever writing showing how an outsider can integrate into a group, going from hostility through grudging acceptance to admiration and friendship.
There are other working class children who are leaders and very much set up for admiration:
Jack in the secret series, Barney in the R-mystery stories, Andy in the Adventurous Four, Jimmy in the circus books etc. All very much working class and very much set up for admiration by the readers.
How many other books written as long ago as those would not trigger some raised eyebrows nowadays?
And I suspect that books written nowadays will trigger just as many raised eyebrows in 70 years time, from things that modern day authors think that they were so forward thinking when they wrote them.