By all means, let it go.

If you happen to go back and it's still there, ask if she needs some help as maybe there's a logical reason it's not done.

I hurt my back last year and doing any housework was hard, but I was too proud/embarrassed to ask for help. My kids did what they could, but I was so touched when my mom came over and cleaned my bathroom and parts of the house that needed some help. I sat and sobbed out of gratitude when she left. Some bathrooms are just mold magnets and within a few days it looks like you never cleaned it.

As long as you are sensitive about the issue, she'll probably understand.