I think he had a bit of a "culture clash" with Graendal/Aran'gar. It was a bit obvious Brandon's personal exposure to erotism, fetischism and such is very limited (while we know Jordan loved his erotica...and liked his food and drink pleasures and other indulgences. Brandon is a mormon - didn't he even have Graendal eat candies at some point? ). Graendal became a bit cartoonesque as a result, and to people with a bit more exposure to sleaze, not believable anymore.

Inconsciously, I think Brandon nay have had a problem (he's not alone, I've noticed that with many readers too) with the whole notion that someone so morally and sexually depraved and self-indulging could also be extremely intelligent and cunning. I think he is one of those readers who didn't see past Graendal's masks enough too. You have to read Graendal's POV very carefully to fully realize just how intelligent she was.

I don't think he tried to dumb her down, I think it just came to him naturally. His perception of the character wasn't as someone as intelligent as "her fans" like us perceive her. He did a bit the same to Cadsuane too.

He hasn't done that to Moridin so far, and I don't expect he will with Demandred.