Any instant web form deployed in the Wolfram Cloud can be used as a mobile app from within the Wolfram Cloud app. Use IconRules to specify app icons for particular mobile platforms.

How to: Create an Instant Web Form describes creating and deploying instant web forms.

This deploys an instant web form under the name "Squaring":

By default, the Wolfram Cloud app uses an icon based on the content of the cloud object.

To set up a custom icon, use IconRules, giving any expression or image as an icon to use:

You can specify permissions for an app deployment; this makes the app public:

Some input types are treated specially when they appear in an instant form within a mobile app. For example, "Image" typically gives access to the camera and photo gallery on a mobile device.