A negative opinion on a dress is not defamation. If it were, then any kind of negative opinion would be in danger of also being defamatory, eg, any 1 star movie/product/service review.
Court cases are costly, any influencer wanting to take things that far would have to fund cases against multiple Tattle members or go after the website itself, as the Sands did.
Unless legal representation was free/kindly gifted. The annoyance stemming from the sheer number of undeclared free stuff is sort of why forums like Tattle appeared in the first place.