NitroPack and CloudFlare are very different services. CloudFlare does only a limited number of web-specific and content-specific optimizations. It has a more general focus on security and performance.
On the other hand, NitroPack optimizes your website, based on all the Google-suggested speed guidelines. It's designed to do one job: make websites fast.
Because they serve different purposes, you can use both NitroPack and CloudFlare at the same time