An earlier outage involving a DNS misconfiguration has lead to some ISPs caching npmjs.com as missing, a workaround for this is to use a third party DNS provider such as
Cloudflare or
Google, the affected caches should clear within the next 24 hours.
If you are using registry.npmjs.com as your registry endpoint, pointing to registry.npmjs.org (the default) should resolve this issue for the time being.