Cloudflare Apps are moderated by the Cloudflare team to improve their quality and safety. Cloudflare Apps attempts to be a safer way of installing tools onto your websites, but that does not mean it is without the risks intrinsic in installing anything onto your website.
The Cloudflare Apps Live Preview provides a safer environment in which you can test an app, but you should always be cautious about entering a password or any other secret information while evaluating a new app. There is also no guarantee that an app will perform identically outside of the preview.
If you have particular security requirements we recommend you evaluate the app's creator and any available source code just as you would with any other tool you install onto your site.