It's not about this picture, it's about using the art in the interface of the Tesla. If you use someone's work in a way that's not covered by fair use, you need to compensate that person.
So, correct me if I'm wrong, but it seems like Tesla cloned the farting unicorn design drawn in their own drawing app, is that correct? If so, how similar it is to the original mug picture? Is it similar enough to be considered a derivative work of the mug?
Both are pictured in the article. The one is clearly a redrawn-from-scratch *direct copy* of the other. You could say it's a different implementation of the exact same design. It's the sort of thing that would generally be okay in the software world (Oracle v Google notwithstanding), but definitely not okay in the context of graphical work. If it had been a clearly different take on a similar idea, then it may have been fine, but as it is, Tesla should have obtained permission and/or compensated the artist for using his design.
And if Mr. Prank can claim a similar drawing to be his derivative, then can I also assume the little pony's author can also claim Mr. Prank's work is a derivative?
Not every drawing of a unicorn is going to be an infringement of another drawing of a unicorn, don't be silly. You can certainly debate where the line ought to be drawn, but if you actually look at the two drawings here it's going to be very hard to argue that this was okay, if you understand what the norms for this sort of thing are.
Anyway, I seriously doubt that Musk was directly responsible for this, so saying "Elon Musk stole. . ." is silly sensationalism.