It is. Your communication to SNS (i.e. the API endpoint) is encrypted using TLS (SSL), and the triggering process (i.e. SNS > Lambda) happens “under the hood” and is presumably encrypted too (ask your friendly AWS SA/TAM for more details).
For an added layer of security you could conceivably encrypt your SNS message payload, but you’d have to do that in your application, as SNS doesn’t have built-in encryption of payloads.