It came as 1 July marked the 60th birthday of the late royal and fashion icon, who died in a car crash in 1997 at the age of 36.
However, some of those watching at home described the speculative segment led by Gok Wan as "ghoulish" and "weird".
The catwalk featured several models including Nicky Lilley who had worked as a Princess Diana lookalike when she was alive.
As Lilley showed off a green and white dress, Wan remarked: "So, I think you can kind of imagine this is what Diana would look like right now if she was still with us."
Writing in response to the fashion portion of the programme, one viewer remarked: "I think this is the worst fashion segment #ThisMorning has ever done. Guessing what Diana might have been wearing today is goulish(sic)."
"Jesus. Who thought up this fashion section of what Diana would be wearing if she was still alive. It’s just bloody weird #thismorning," another shared to Twitter.
A third wrote: "@thismorning Cringing throughout the Poundland Princess Diana fashion segment. Love Gok, but the styling & tenuous concept was dodgy. #thismorning."
"Not too sure about this fashion segment on @thismorning today. What Diana would wear today...just a tad distasteful. #ThisMorning," a fourth viewer said.
Earlier in the programme, hosts Phillip Schofield and Holly Willoughby had heard from Lilley on what life was like as a lookalike.
She said: "Yes I used to do lots of fun things, different jobs all around the world. It was a wonderful few years when I working as a lookalike."
Thursday will see Diana's children Prince Harry and Prince William unveil a statue in her honour in Kensington Palace's Sunken Garden.
The occasion will also be attended by members of the late Princess' close family.
Watch: Tributes placed at Kensington Palace on day Princess Diana would have turned 60