The idea that Marinette's salt/bashing fics are far more numerous than they appear because they are untagged as such is an interesting argument.
A: trying to counter it would mean manually reading through every Miraculous fic on AO3- which is functionally impossible, or relying on personal anecdotes which can be easily countered with someone's else's or dismissed entirely.
And, B: The same argument can just as easily be made for Adrien, Alya, and even Lila.
And if you'll deign to accept my own personal anecdote- that's very much the case. Demonizing Adrien and Alya is so common that it's not even mentioned half the time. That demonization is flat out accepted as an axiom of Maribat/Chameleon sub-genres to the point where the story doesn't tag otherwise then it should be expected as surely as the sky is blue.
And it's far from contained within those sub-genres. The sheer vicious glee that was taken in savaging these characters to prop up the idea that Marinette lived a life of hardship can hardly be overstated. Ron The Death Eater is very much in effect, and to the point where large swathes of the fandom take it as a given that Adrien and Alya are at best pathetic, naive, or just fundamentally lesser than Marinette. The same applies to the class as a whole, albeit to a lesser extent.
Even from the start she had issues and traits that could be salted on, characteristics that the show insisted on glossing over, or presenting in a comical or softer light so that they'd be forgiven or dismissed. Things that were ripe for deconstruction and harsh critique if taken seriously. But if you were to search for stories published before 2020 you'll find scarcely any stories that punish her for her thieving tendencies, the behavior that causes so many Akumatizations in those around her, or just her objectification of Adrien.
And to go on a related tangent: let's just say that there's a very good reason why roleswap fics don't ever have Mister Bug hitting Lady Noire with his yo-yo the way Ladybug hits Chat Noir.
There is a clear double-standard in play here. One that is very much in favor of Marinette's character.
But let's go for a more measurable example.
One of the great staples of Adrien salt is the perennial stripping of his Miraculous. It's justified in many ways, but the ultimate point is that he is unworthy of it for one reason or another, usually based on his canonical actions. This can be due to his character, alleged incompetence, or culpability in allowing Hawkmoth or Mayura to escape and continue their reign of terror.
But what he has never done, and what Marinette has done twice, is lose the Miracle Box to the enemy. The second time can debated as to whether or not it is her fault, but the first time is undeniably solely on her and her petty jealousy- as are the consequences.
This alone is worse than anything Adrien has done in any identity. And it was far from her first offense of risking the Miraculous over petty reasons ("Volpina" for a single example). And yet she was rewarded for it with authority and free access to over a dozen Miraculous. And for all the show insists that it's a terrible burden and responsibility, it doesn't outweigh the fact that she was promoted for her failure.
A failure whose severity easily justifies the revocation of her Miraculous and any memories that she might have relating to it. At least if held to the same standard that Adrien, Alya, or any other character in that class the collective fandom holds them to.
But, if you were to search for stories written before the season five release date (October 8, 2022) that featured Marinette being stripped of her Miraculous for this action, or even being addressed in a way that was anything but sympathetic by the narrative, you'd be hard pressed to find more than a handful. In fact, I'd be greatly surprised if you could find even one which genuinely salted on her for it in the same way that Adrien and Alya were castigated for so much less.
