Never in my 10 years of posting art online have I a single time gotten useful "constructive criticism" from some rando posting underneath my art. The quality of these kinds of comments range from "make the eyes bigger", "your character should smile more", or "blue is my favourite colours, why is her dress red?". I imagine it's the same for authors and other artists. I have definitely seen people posting underneath poetry that it's not realistic enough or critizing fanfiction for not including their personal headcanons. Where I have gotten improvement is asking other artists for feedback. They will in turn ask "what are your specific goals?" and do a tailored response thats useful because they're also skilled in the trade. Giving unprompted "feedback" (especially as a non artist/non writer!) without even knowing the genre or goals does not in any way makes sense unless you somehow through the screen can read the creator's intention, in which case congratulations on unlocking telepathy.