Critical CSS can definitely help how quickly your page appears to load, by giving users an immediately-available above-the-fold view.
However, of all the PSI fixes, this is one of the most complex components to maintain. It may be easiest for you to skip this recommendation, and many clients have chosen to do so over the years.
We often recommend compromising by leaving the theme stylesheet render-blocking, then deferring all other styles. This prevents the need for maintaining critical CSS but may still have a small impact on PSI scores.