She also told The Telegraph: "The sales have been dreadful, and I hear that the people at Penguin are virtually slitting their wrists over it. To think of all the money they spent on it, too!"
Middleton pocketed a £400,000 advance from Penguin for the book, which is described by the publishers as "packed with recipes, crafts and inspirational ideas".
The 416-page glossy hardback, which features photographs of Middleton engaging in party planning activities like cooking and decorating went on sale for £25 but has since been slashed to £10.99 on Amazon.
But the public seem less than enamoured with her efforts, with Celebrate shifting just 2,000 copies in its first week and sinking from sight on sales lists.
The damp squib response of her efforts has put Middleton somewhat on the defensive.
Cooper (right) adds: "I don't blame her, I think she was ill-advised. All I can say is I hope the British public don't demonise her because of it.
"She seems like a lovely girl and her sister is wonderful."
Middleton's labours have also been lampooned via a genuinely funny Twitter parody account entitled Pippa Middelton Tips.