During a lecture at the British Museum, the double Booker Prize winner criticised Kate as appearing to have been "gloss-varnished" with a perfect plastic smile in contrast to Princess Diana, who she described as awkward and emotionally incontinent.
She went on to suggest that the "painfully thin" Kate was selected for her role of princess because she posed no risk of showing any character.
Mantel's remarks were made two weeks ago during a lecture at the British Museum, organised by London Review of Books (LRB), a month after her latest novel Bring Up the Bodies won the Costa prize.
She said: "Kate seems to have been selected for her role of princess because she was irreproachable: as painfully thin as anyone could wish, without quirks, without oddities, without the risk of the emergence of character.
"She appears precision-made, machine-made, so different from Diana whose human awkwardness and emotional incontinence showed in her every gesture."
Mantel, whose latest novels are set in the Tudor court, said she saw Kate becoming a "jointed doll on which certain rags are hung".
She added: "In those days (Kate) was a shop-window mannequin, with no personality of her own, entirely defined by what she wore.
"These days she is a mother-to-be, and draped in another set of threadbare attributions. Once she gets over being sick, the press will find that she is radiant.
"They will find that this young woman's life until now was nothing, her only point and purpose being to give birth."
She also blasted Kate's first official portrait by Paul Emsley, unveiled in January, labelling her eyes as "dead" and wearing "the strained smile of a woman who really wants to tell the painter to bugger off".
During the lecture Mantel went on to question whether the monarchy is a "suitable institution for a grown-up nation" in a society which sacrifices royal ladies and allows them to be entertainment.