If you gave the goodest person in the world a billion dollars, you'd technically have a good person who was a billionaire, so my declaration would be wrong. But only for as long as it takes them to get rid of it.
I don't know about Melinda Gates (literally haven't heard anything), but MacKenzie Scott does seem to be burdened by the money at least somewhat and has been shoveling it away.
But if you compare, JK Rowling couldn't bear being a billionaire, and it turns out that she's a world-class asshole. Maybe if you're MacKenzie Scott rich, it's just hard to give the money away quickly, and we'll see her end up with less than a billion however long that takes. Otherwise, I think she's got to be at least as much of an asshole as Rowling.