Do not add "postinstall": "copy-dsfr-to-public" in your library's package.json. It's the responsability of the host app to do so.
If you rely on MUI, @mui/material@emotion/styled and @emotion/react should be peer dependencies as well. You should add those modules as devDependencies.
If you use TSS: react-dsfr dosen't need to be a peerDependencies but @emotion/react does, you do not configure the emotion cache in your lib, that's the the responsability of the host app.