Like her mother-in-law, the late Princess Diana, Middleton, 29, has already become a style icon in her own right.
Palace watchers expect Kate's wardrobe for the U.S. and Canada tour to include three dress changes per day, amounting to 40 outfits in all for the young bride.
"Every outfit will be closely analyzed," Nicholl told "GMA." "But she's made a decision to travel without a stylist or lady-in-waiting."
While Diana famously drew from designers and fancy gowns, Middleton has gone for more affordable and accessible wear, including a Burberry trench coat that sold out as quickly as photographs of her in it surfaced.
The dress she wore to meet President Obama and the first lady last month in England was so popular it crashed the store's website for more than two hours.
Nicholl, author of this month's Vanity Fair cover story on the royal couple, told "GMA," however, this trip, for Middleton, will not just be about her style.
"It's not just about what clothes she wears but what she's capable of doing, of fitting into the royal role," she said.
"She is an ambassador to Great Britain, and what better opportunity to showcase this than a trip to North America."
Planned stops on the couple's busy Canadian itinerary include places of personal importance to Middleton.
During their visit, which is being paid for by the Canadian government, the newlyweds will tour the Canadian airfield where Middleton's late grandfather, Peter Middleton, was based as an RAF pilot during the Second World War.
He died at the age of 90 last year, a time at which the couple delayed the announcement of their engagement so Kate Middleton could attend his funeral, according to the Daily Mail.
They will also make time for a more lighthearted moment when they visit the country's smallest province, Prince Edward Island, the location featured in Middleton's favorite childhood book, Anne of Green Gables.
The couple will also help open the Calgary Stampede, the world's largest rodeo, on their first tour together.