The joke works. It just ain't funny. Context matters. It's not just because Vivian is trans. It's because Vivian is not one of them. She's disowned and disenfranchised. If Vivian was a hypocrite and in line to inherit any money whatsoever the joke might have landed. But it doesn't. No big deal. That's the nature of comedy. A lot of it misses the mark. Like this meme.
To be fair I think Vivian's picture was used because she is the only recognisable child of Musk, and also because I think most of his other children are still literal children. There is probably no ill intent from the original post and I doubt OP shared it for transphobic reasons. The joke should land, but it doesn't. At least not for anyone who recognises Vivian in the picture and knows what she has been through. That's the nature of comedy.
Sure but I think this joke is also leaning into the Trumps being transphobic and including Musk's trans daughter (sorry I forget their name). So the joke also comes off as transphobic in a way.
Always has been.