While the applicability of the content of the essays can vary quite a bit* - "Blind and Dumb Criticism" being all but required reading for anybody wishing to call themselves a critic, and "Operation Margarine" being a nice little rejoinder with which to shut up the "Don't you get it? I'm supporting the workers by buying Hogwarts Legacy" crowd - the form underlying these essays remains unassailable. It's not about the particular results Barthes arrives at, but the critical attention to detail he shows in demonstrating how exactly a given system of signification can signify. The worst criticism you could levy against Mythologies is that, in 2022, the idea that we can see through the falsehoods holding up this modern culture of ours - and in fact our being encouraged to do so - is a substantial part of what sustains that culture, and has been for a long while now. Even then, a little care in how we approach seeing that culture couldn't hurt.
*No doubt some of this comes down to Barthes' writing being so incredibly specific to his own historical and social milieu and our own milieu being so radically different from his that there simply isn't much left when you transplant the former into the latter. How much advertising, toys/children's entertainment (hello Gainax! Hello Fortnite!), and the social standing of writers have changed since Barthes' day is ample proof of that.