It's not ridiculous from a PR point of view.
Look, you cannot use working class sloganism throughout your politics and to gain popularity and then send a totally discordant message with your clothes branding in public facing roles.
As has been pointed out better than me, fashion designers inhabit the realm of social politics too. Whom you choose to wear, how eco conscious their clothing is and how much it costs absolutely isn't a neutral choice.
When Meghan Markle was touring the country with Harry whilst engaged, they still retained decent PR at that time, and either she herself or someone advising her would select local designers in the area they were visiting for her to wear, I particularly remember her wearing Jeans from a small eco-conscious not well known, home grown Welsh brand. It was skillful PR. I think she also wore Marks and Sparks a couple of times.
So yes, criticism of which designers the wife of the PM favours, when said PM has been deftly signaling his WC affiliations and sympathies matters.
Purely from that pov, (not her body) it was a big faux pas and a missed opportunity.