Is my stolen's iPhone data safe?

Not unless they know your iPhone passcode or your Apple ID and password.

BUT: Beware of a common phishing scam. You may get a text or email from “Apple” or “Apple Security” (or sometimes “Police”) saying your phone has been found and to click on a link to see its location. This is NEVER from Apple or from law enforcement; it is from the thief or fence to steal your Apple ID and identity. Delete any such message without responding to it. Apple never gets involved in lost or stolen devices→Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams

Here’s more guidance from Apple regarding what to do if your phone is stolen—> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support