There's nothing wrong with using Carbon or any other FOSS design system. I would encourage this practice for Nostr projects as we lack designers, and developers are moving blazingly fast. Hundreds of designers worked on Carbon; it's a rock-solid product with tons of documentation. When you have isolated components in a storybook.js, it's super easy to just pick them and build your product like a Lego. You can distinguish your product with different color schemes and branding. Eventually, you can fork the storybook repo and make smaller or bigger visual changes here and there to make your product visually unique.

I've played with Carbon over the years but have never used it in an actual project. It has a very distinct tech-ish style. If you are not going for tech style, you will do better with most other design libraries with a more neutral style that is easier to iterate on.