I saw a post going around about costube historians analyzing period film costumes for accuracy and it kind of got under my skin, so I'm sitting down and writing ... not exactly a response to it, but a discussion of the topic.
(It would be a direct response except that I don't actually watch costube, because quite frankly I can't watch/listen to people discuss things I already know. And I don't want to be like "they don't do X!" when maybe they really do X and I'm just not aware. But a lot of the complaints hit the same points that have been brought up against fashion historians for reviewing costumes for decades. I would also note that I have looked into specific videos where there were claims of terrible costuber behavior and watched them and found nothing.)
If you're going to analyze a period film's costuming in any way, you should still interact with the historical aspect to some degree. If you want to talk about the use of bold stripes in Tim Burton's Sleepy Hollow, for instance, and you don't mention that they were in style during the period the film appears to be set in, it's kind of weird.
Likewise, yes, if you're critiquing primarily from the angle of historical accuracy, you should also engage to at least some extent with the reasoning behind the inaccuracy. If a reviewer doesn't do so at all, then yes, their review is probably not as good as it could be.
People pointing out an inaccuracy (or many inaccuracies) are not inherently scolding the costume designer. Even if their tone is something other than sweet. Sometimes they are scolding other people involved in the production, like the director who mandated a particular costume, or just a general notion of TPTB. Usually they are divorcing the art from the artist, though, and just reviewing the costuming from their particular viewpoint and knowledge base for a likeminded audience.
Sometimes, yes, they are complaining directly about the costume designer. This is not a crime. Some costume designers (for instance, Sandy Powell) have an incredible grasp on fashion history and excellent taste when it comes to diverging from it. Others simply don't have as in-depth of an understanding and make design decisions sometimes based on stereotypes and myths. Some costume designers will explain their decisions in interviews or blog posts and make it clear that they didn't make a truly informed decision about accuracy because they didn't know enough about the period. It's important for both sides of the equation to stop painting the other with too broad of a brush ("ivory-tower elitists who have no idea of a production's needs or budget" vs. "costumers who know how to sew but not how to do historical research").
If you're allowed to complain about a writer or a director or an actor doing something you don't like in a movie, you're also allowed to complain about a costume designer. You're allowed to have aesthetic preferences, and even to talk about them without hedging every five seconds to make it clear that others can disagree, although some of this is beneficial with any critique. Why would it be otherwise?
This seems really obvious to me, but maybe it's not? But "they costumed that female actor in an anachronistically sexy way because sex sells" is a feminist issue. The assumption that women's bodies should be sites of less-clothed allure while men's should attract by being more covered (with more layers than in modern dress, with cravats, etc.) is sexist. Complaints about female characters being costumed inaccurately are often being made along these lines, and pointing out that the producers insisted on it or something does not mean it's suddenly unproblematic that every female character deemed fuckable has to have low necklines at all times and modern shiny hair.
It's true that fiction isn't non-fiction and shouldn't be taken that way, but it's also demonstrably true that viewers do take cliches in film aesthetics as accurate when they see them enough times. People cite Scarlett O'Hara's 18" waist. They believe there were no bright colors before the 1920s and that women couldn't have put their hair up unless they were wealthy. These beliefs have consequences when it comes to public perceptions of history, and if films perpetuate them it's perfectly reasonable to point out that they support ideas about e.g. gender roles that trads express today.
It's also simply funny when a film's hair or costuming or makeup is supposed to evoke a lack of artifice but actually requires quite a bit of artifice because people don't naturally have perfect hair and skin and so on.
If you don't like reviews of period films that focus on the accuracy of the costuming, maybe ... don't watch/read reviews by fashion historians and historical costumers? At least unless they're vetted for you by someone who doesn't mind that?