I want to tell a woman she has zits all over her butt. Do you think this is a gross topic?
Really? It is kind of gross now that you mention it. How do you plan on going about this act of telling her though?
No plan, just say it out loud in front of her. No problem.
I think that you should reconsider. It is not cool to make someone feel insecure about the way that they look.
It's no big deal plus, it can lead to a good discussion about beauty standards.
Appearances are not all that, it is childish to think beauty has specific standards. You should just help her bathe, and be caring. Make sure she gets positive attention and not insults.