It's a "better than nothing" option, as there are a slightly higher number of actively exploited attack vectors that apply to the client to CDN connection than the CDN to origin server, such as "free" wifi that injects ads, malicious ISP DNS, and the like. But it's not actually secure, as the origin server to CDN connection could be tampered with, and just because there are fewer active attacks that would be likely to affect that connection right now, doesn't mean that someone won't come along later and hijack such a connection.
CloudFlare offers other TLS options that do include encryption and authentication between the origin server and CDN, but they do require that you set up a certificate on your server, so if all you're trying to do is enable TLS (and don't care about the CDN), just installing a cert on the origin server and using TLS is probably a simpler option that using CloudFlare.