The fashion guru - one of Kate's go-to designers - told Vogue: "She's highly attuned to those details that will provoke a tabloid frenzy, and above all, she does not want a tabloid frenzy.
"She doesn't seem to feel the need to dress sexily to woo the world. She's dressing for William and herself."
Mouret said that the Duchess shunned bottom-hugging dresses - specifically a cream dress she wore to Claridge's last year which was cut looser than the sample fit - so as not to make a rear fuss.
He revealed: "She doesn't want a bottom situation like Pippa had."
The designer also claimed HRH currently has less fashion power than Kim Kardashian.
Speaking of "The Kate Effect" - which can see an item of clothing sell out in minutes - he said: "You don't see hundreds of women running around looking like Kate.
"The kind of people who like Kate's style are not the kind to rush out and copy her dress. To be honest, we're more likely to get orders on a dress that Kim Kardashian's worn."
It is thought that the Royal fashion phenomenon has lost some appeal because Kate often favours wearing last season's designs which are no longer accessible.
And it's not just high street stores who have benefited from "The Kate Effect".
Littlewoods reported a 238% jump in sales of hockey sticks after she posed on the pitch last year at the Olympic Park.