Magical and Alien Diversity
This trope has often been criticized, and rightly so. Partly, this can be problematic because readers hoping for representation get faced with a character who's magical or nonhuman being the only one like them in so many stories, which is frustrating and alienating. It also often comes from a place of cowardice for the writer - many writers use magical or nonhuman analogues because they're afraid of misrepresenting the real-life characteristic, or because they don't want to face the true challenges faced by mundane humans with that characteristic. And lastly, it can be linked to the desire to make every 'nonstandard' character trait be plot important - you can't just have a minor character who happens to be having a psychotic episode, instead, they have to be psychic so their bizarre ramblings are plot important.
I agree with those issues, and would love to see more characters in speculative fiction who's minority identities have nothing to do with whether they're human or not or what magical features they have (like Nora and Mary Lou in Vampire Diaries, who are both vampire siphon-witches who just happen to also be lesbians). But even so, I still like the magical and nonhuman portrayals of human forms of diversity. There are some interesting things that can be done when you explicitly link a particular feature with a magical or nonhuman category.
First, it makes it easy to explore the idea of a society where that trait is the norm. The easiest way to explain having a culture where everyone is intersexed and/or agender, for example, is to simply introduce aliens who reproduce in a non-gender-dependant manner. A species of blind underground trolls could be used to explore how a society is built without sight.
Secondly, while deciding to avoid the real-life equivalent so you can avoid getting stuff wrong can be cowardice, it can also be quite practical. If your story requires something to happen that we don't know how to do in real life, or that physically can't happen in real life, making them a fantastic analogue avoids the fridge logic moments for those who know how these things work, and can avoid getting your readers derailed by controversial discussions that you'd rather not get involved in. (An example would be the story idea I had for some kind of magical event causing the majority of children born shortly afterwards to turn out gay.) It can also lessen the risk that your story will become dated when new research changes our understanding of a minority group's characteristics.
Thirdly, stories with fantastic analogues might draw in more prejudiced readers who would never have read that story if the character was seen as part of a real-life group they dislike. While we certainly shouldn't censor all of our stories for the sensibilities of prejudiced people, a story about a shapeshifting race with no true gender might get read by someone who has no desire to read a story about a non-binary human character. What's more, a children's story with these creatures might find its way into the hands of a child who has no access to representation of realistic non-binary characters, because such characters are seen as unsuitable for children by the adults responsible for them. Fiction can have a great power to open people's minds, especially if it's not obviously intended to communicate a message.
And there are many interesting things you can do with this trope. One Star Trek episode I saw focused on an alien race where most members were non-binary, and the few who had binary genders were subjected to reparative therapy to force them to conform to the norm. I'm currently working on a story setting with vampires who are aromantic asexual but feel what can best be described as 'parental attraction' towards people that they want to turn into new vampires. I also wrote a short story once about a species in which children were universally psychopaths until they hit puberty, allowing me to explore how a parent would guide a child through the experience of feeling empathy for the very first time. None of those stories would have worked if the characters were actually members of the real-life minority they were designed to resemble.
And lastly, in a setting where non-marginalized traits are fair game for being linked to magical or nonhuman characters, why not have marginalized traits be treated the same way? In a setting that has aliens who can assume any form and spend most of their time mindlinked to each other, it would be unfortunate if the creators felt that making a species genderless or mute was out of bounds.