Are we speaking of cartoonists and not "artists" (whatever that means) in general?
I'm afraid I'm giving my age away, but I remember it was said of the
Li'l Abner comic strip that all the characters looked like caricatures of cartoonist Al Capp.
en.wikipedia.org
So it's not unprecedented or peculiar to trans artists.
Does Sophie just not read much of anything? No matter what you read, there's a pretty good chance that the authors experience will bleed through at one point or another, that's just how it works. The reason it gets more focus with LBGT+s and women and POCs and the like is because, let's be real here, their perspectives were either ignored or taboo until recent generations, and now it's kind of 'hip' in scholarly setting to be all about that. It's just that right now, the fact that an artist was a woman, or gay, or trans is the selling point for stuff. At least with the growing trend around 'writing about my experiences as *blank*'. And that's not bad, I like those things as well. It's interesting to learn about experiences different than mine, but don't be surprised when I see that the main character is the author.
And it's not ignored with cis white guys either, it's just usually about stuff that isn't about gender or sexuality because guess what, there's more to life than that. Jobs, hobbies, travels, areas of expertise, taste in food and music, all of the hundreds of little things that make up someones experience can find their way into their art. Stephen King is a writer who used to drink, like a good amount of his characters. Tarantino likes blaxpoitation and feet, as evidenced in his films. Douglas Adams loved technology and computers, and it shows in his writings. George R. R. Martin is fat, and he loves to describe food. We could go on forever, but you get the idea.
Separating art from artist is a good thing. If a work can't stand on it's own without us knowing about the author, than it's just not a good piece. However, that is not to say that knowing about the author can't bolster, or even change, the work by a large amount. Sonichu would not be as worth reading if you did not know about OPL. Knowing about Frank Miller's tastes can help
Sin City explain itself more. Knowing just what the fuck Tommy Wiseau was doing behind the scenes turns
The Room from an unintentional masterpiece into a work of godly proportions. Knowing that Chuck Palahniuk is gay can cast a different light on his works. Knowing that James Clavell was actually in a prison camp makes the events of
King Rat much more real to the reader. And so on and so forth, blah blah blah.
Basically, Sophie's a tard who seems to only see the world through a lens of gender and sexuality.