Instruction in giving constructive feedback definitely helps. I've done a few writing workshops and the guidelines they give are priceless. But it's also stuff that (at least in principle) is pretty simple - thinking about whether the author/artist achieved what they set out to do vs. whether it fits your own preferences ("Does this version of Harry and Draco's relationship really lend itself to bondage? Did you set out wanting to communicate the origins of that desire/relationship, or do you want to plunge the reader right into the action?" vs "bondage is gross and H/D would never do that"), giving feedback that's actionable ("This description was confusing - could simplifying it help?" or "I didn't understand this character's motivation. Where do you see it coming from? Would giving that reader the backstory be helpful?" instead of "The things your characters did really didn't make any sense."), and owning your own subjective perspective ("I was confused by x and y" instead of "x and y were incomprehensible and would be impossible for readers to understand"). But it is totally learnable!

That said, tbh, I'm nervous about giving feedback to artists. There's so much I don't understand about the process, both the decision-making and the technical aspect, and it's definitely going to take some learning from me. I hope it'll make me a better commenter post-concrit and I think for me that will be worth it, but I'd be lying if I said I didn't have some nerves about it.