I guess it's just a matter of opinion, but I don't think the musical is trying to make them, or Miss Trunchbull, for that matter, truly awful. Yes, they're vile, but they're also incredibly stupid and their stupidity is on constant display through the entire play. While I wanted Matilda to win against them, I still found elements of comedy in them.
And I mean, it does kind of matter that the creators are presenting them as they are in the book since they are, you know, adapting the book. To change them substantially would, I think, pretty seriously alter the material from Dahl's book. He wrote nasty adult characters and intended them to be nasty.
I am a firm believer in serendipity- all the random pieces coming together in one wonderful moment, when suddenly you see what their purpose was all along.