Lady Boothroyd said that Mr Bercow, who has instead chosen to wear a simple black gown over a business suit, was "letting the side down".
She said that she had confronted Mr Bercow over his rejection of the Speaker's waistcoat, cuffs and winged collar.
"I said 'nurses like their uniform, boy scouts, girl guides – you've 700 years of history behind you’," she told the BBC.
Mr Bercow replied that "he didn't feel comfortable" in the traditional attire, but she said: "I just think you're letting the side down a little bit."
Lady Boothroyd, the Speaker between 1992 and 2000, herself dispensed with a long-bottomed wig worn by her predecessors.
She continued to wear tights and knee breeches that were eventually done away with by her successor Michael, now Lord, Martin.
She also said that she was "appalled" to read in the Daily Telegraph this week that MPs were being given secret expenses repayment deals by the parliamentary standards commissioner.
"I am appalled if that is the case that this can be done behind the scenes – you pay up and we won't name you.
"We have a right to know, absolutely I agree with that. No, there should be no hiding there at all."
The "rectification procedure" was introduced in an amendment to a parliamentary standing order in July 2005.
It states that MPs may come to a private arrangement with John Lyon, the standards commissioner, if they agree to repay the money involved.
Mr Bercow has promised that the rule will be scrapped and the names of MPs repaying expenses will be published.
Lady Boothroyd said that she thought Mr Bercow was "making a good stab" of the job.
