I get why people are drawn to drawing mature-themed art of public figures — it's expressive and can push boundaries — but the legal picture is more layered than it seems.
From a copyright perspective, the original fan art you create belongs to you as long as it's genuinely your own work and not a trace or direct copy of a copyrighted photograph. Likenesses of public figures aren't copyrighted, so drawing Madison Beer from memory or reference is typically within your rights. However, if you directly copy a photographer's image or heavily reference a copyrighted photo without transformation, you risk a DMCA takedown because the photo's copyright owner can claim infringement. Beyond copyright, there's the right of publicity: many places let celebrities control commercial uses of their image. Selling prints, using the image on merch, or promoting products with her likeness can trigger publicity claims, depending on jurisdiction.
Platform rules add another layer — many social sites and marketplaces ban sexualized images of real people or have strict rules for NSFW content. Even if it's legal, it might get removed or lead to account strikes. My approach is to keep my work clearly original, avoid exact photocopies, label NSFW pieces, and shy away from selling anything explicit without permission. If I ever planned to monetize those pieces, I'd consult a lawyer or avoid it altogether — it keeps me out of trouble and frees me to enjoy the art-making part.