This isn't a big issue and more like an optimisation, where we could only append to the stylesheet the current classNames being used.

We could introduce Stylesheet.collect to analyse the html as a string, and compare the classnames.
Making sure the "critical" is the required to be appended to the resultant HTML.

let stylesheet = Stylesheet.get_all();
let html = ReactDOMServer.renderToString(<App />);
let (critical, other) = Stylesheet.collect(html, css);

Taken from: https://github.com/callstack/linaria/blob/master/docs/API.md#collecthtml-string-css-string--string

The API might change, but the solution should look quite similar