I've got the time, so I'm going to treat this as serious question rather than a rhetorical one.
This post reminds me a lot of conversations we used to have about fanfiction and feedback in the 1990s. At the time, at least among the people I knew, there was a sense that we wanted fanfic to be taken seriously and that it was important for that reason for fan readers to give fan writers constructive criticism so that they could improve their writing and therefore support the argument that fanfiction was worthy of the vast investment of time and energy that we were all making in it.
Since that time, the following things have been established:
Fanfiction will never really be taken seriously outside the community of people who write and read it. It doesn't matter how many well-constructed arguments we make about how the Aeneid is fanfiction, etc. Fanfiction is not devalued by The Outside because of the quality of the writing. It is devalued because a) it is unmonetized and unmonetizable; b) it is largely created and consumed by women; c) often, the sex is the point. Even professional and profitable approximations of fanfic (50 Shades, Heated Rivalry) which command attention because of the money they make are devalued. Most mainstream media coverage of the Heated Rivalry phenomenon, for instance, is explicitly or implicitly answering the question, "How has a show that Can't Be That Good generated such a large and passionate fan base?"
Most fan readers aren't equipped to provide criticism which is useful to the author from a technical point of view. In my experience, criticism in AO3 comments is usually either the expression of a personal taste (I don't like stories where X happens), an attack on the writer's presumed beliefs/ideologies, or the airing of a generic pet peeve (you broke my favorite writing rule). The kind of critique you would get from a writers' workshop, in which the focus is on making the story better through revision, is rare and honestly, not part of the reader's job. If you want that kind of criticism what you need is a writers' group where everyone has agreed to do this work for others, so that others will do it for them.
No attempt to raise the general quality of fan writing has had any significant impact or staying power. And my theory is that this is because
Most readers of fanfiction don't care much about the quality of the writing. For some readers a certain level of technial skill is a basic requirement. But for a lot of readers, what they want is more of whatever they're consuming, and if it 'works'--if it produces the desire emotional/sensory effects--then its 'quality' is irrelevant.
Thus it is true that a lot of positive feedback on AO3 is instrumental rather than evaluative. It is often either a veiled attempt at marketing (readers praising a story because they want other people to see it and enjoy it) or a veiled attempt to encourage the writer to keep producing more fanfic. At the same time
It is also true that people wouldn't provide positive feedback, even for these reasons, if they weren't getting pleasure from reading it. So from that POV, the kind of positive feedback you're talking about is sincere, even if what is being appreciated is not technical skill.
To the question of "what can anyone learn from this unearned positive feedback?" I would answer: You learn that someone liked your work. I personally find that both cheering and useful. If the commenter is sufficiently enthusiastic you might find out *what* the reader liked about your work, which I find even more cheering and useful.
To the question, "Why bother improving when garbage rakes in the kudos?" I would answer: For the same reason most writers improve: because they want to. Or, to put it more idealistically I guess, for the love of the game.
Because here is the thing: many of us, especially Olds such as myself, have been taught that in order to become professional writers we have to improve our writing technically. Many editors do have a preference for distinctive voices, elegant style, complex plotting, etc., and also have Secret Style Rules that authors are not allowed to break. So we're used to the idea that you should improve your writing because it will make you more attractive to literary agents and editors and thus bring you closer to the dream of getting money and fame for your writing.
Whether or not things ever worked that way, they don't now. Look at what just happened with Shy Girl. Or things like the Left Behind series, or 50 Shades. Popularity and salability are not necessarily or even primarily linked to what critics and academics would consider good writing. Good writing as defined by critics and academics has always been something important to a relatively small minority of the reading population. Misconceptions about this are maintained partly because academics, who value innovation and complexity, select for those things when they teach literature. The number of people reading James Joyce's Ulysses in the 1920s was much smaller than the number of people reading, say, Paul de Kock, a bestselling 19th century French author whose trashy novels are name-checked in Ulysses (because Leopold Bloom's wife Molly likes to read them in bed).
So why bother to improve your writing? Because you want your writing to be better. That's the only deep and durable motivation any of us ever really have. Whatever reward you get from your publishers or your readers--whether it's money, fame, or comments and kudos--is as much a matter of luck, connections, and self-promotion as it is the quality of your writing.
I mean I absolutely understand the frustration here. It's very frustrating to put in the amount of work that, say, I've put in on my AO3 fiction and watch other writing that doesn't have any of the technical features I spend so much time creating get orders of magnitude more positive attention. It was even more frustrating to write what I thought was a pretty good original series and be unable to move it past the gatekeepers of what was even then a shrinking and increasingly profits-driven publishing industry. None of that is the fault of your readers, though. People liked your work even though it had flaws? Enjoy that! And if you see in your earlier work, or your current work, things you want to improve, improve them! It may not lead to increased readership or more meaningful comments, but it will satisfy your own desire to be a better writer.
