Domain registrar has nothing to do with the hosting. As long as you can point a domain to an IP address you can get a certificate. Namecheap even goes a step f

Domain registrar has nothing to do with the hosting. As long as you can point a domain to an IP address you can get a certificate. Namecheap even goes a step further and has an API for domain validation (makes LE certificate authorization easier) so they are very friendly in regards to Let's Encrypt.