Thank you for the information, and all the insights!
This all reminds me of how people say "ol' Billy Shakespeare" when they get silly. Even more strongly, it reminds me of when I was like 11 and I hung around online with Sailor Moon fans all the time, and we called the creator Naoko-Hime (hime meaning "Princess").
Would Pamela Colman Smith have liked it, knowing we swiped her friends' nickname for her? I have no idea. I can definitely see it going either way.
The only thing I can figure is, if random people in the world were calling her Pixie and otherwise ignoring her, that would be pretty bad. But random people in the world don't know or think about the artist behind classic tarot cards at all. Which is also bad! Meanwhile, the only people who do call her Pixie are also the ones who know her real name, know at least a little about her life story, and have studied her work. So the idea of those people referring to her with that kind of warmth feels pretty appropriate. To me, it doesn't feel creepy--like, I don't know, when people who dislike Hillary Clinton call her Hill. Like I said, though, I can see the argument the other way.