By certbot key I mean letsencrypt key, sorry for confusion. That means that you need to hard-code the public key of the certificate that you're using in the cl

By certbot key I mean letsencrypt key, sorry for confusion. That means that you need to hard-code the public key of the certificate that you're using in the client application. That certificate might be signed by letsencrypt or any other CA, it doesn't matter. If you're pinning CA key, you put trust into entity which you don't control. They can replace any of their keys. They can stop signing your certificate. It makes little sense to pin CA certificates. Just make sure to have a plan for key rotation in case you need to revoke your key.