Secret indulgences do not have to be beauty related. As I have said, it is something you do for yourself because it makes you feel good. Shopping, crafts, travel, any of these can be a secret indulgence. It is good to have something that makes you feel good. When you feel good, your inner diva feels good, and you will have a feeling of greatness about you.
I know many are mothers and some will say my children are my indulgence. It is nice to indulge on a child, but you need to indulge on yourself. I have seen where women are so indulgent on their children, they forget, they are an adult. More importantly, they forget they are adult women. There is nothing wrong to stop and take a moment and indulge on you. It helps you so much. It also gives you a "me" moment, which can help you with stress of being a mom.
So yes, it is ok to have a secret, a secret indulgence that is. Don't expect to explain why it makes feel good. Just enjoy that it does. Let yourself and our inner diva feel good.