Two thirds of the French now disapprove of Miss Trierweiler, who moved into the Elysée Palace with her partner, President Francois Hollande, in May.
Their disaffection for the 47-year old Paris Match journalist has been attributed to her confusing relationship with the Socialist leader and her public feuding with his ex-partner.
In her first interview since leaving the Elysée six months ago, Mrs Bruni-Sarkozy suggested the twice-divorced Miss Trierweiler become the wife of Mr Hollande, who has never married.
"I think it is simpler to be the legitimate wife of the head of state rather than being his partner," she told the new edition of French Elle magazine.
She added: "Maybe I'm wrong and their choice is a modern one. But for my part I felt a real easing of the general worry about me when I married Nicolas."