While I believe that nobody should be able to wield that much wealth in the first place, arguably being a world-famous artist is among the least unethical ways to become rich as long as the artistic work isn't bought from ghostwriters. It takes $10 in net profit from 100m fans to make a billion, and I could easily imagine a sizable crowd of her followers genuinely wanting to give her money like that (whereas, for example, I never would have willingly agreed to give Nestlé's leadership money, even when I used to buy their brands still).

I don't care about her specifically, but from a "owning the fruits of your own labor" perspective, I think it makes sense for the art itself to be a big part of that.