As of right now, there is no out of the box way of displaying a string directly from a microflow on a page.
An easy way to solve this is via the use of what we call a ‘helper’ object.
You can add a non persistent helper entity in your domain model. Add a string attribute to your helper entity. Then you can either create this helper object in a data view widget with a microflow datasource on the page itself, or create it when opening the page and pass it to your page.
In your microflow have an object of your helper entity as a input parameter, instead of creating the string variable you can now change your helper object and set the string attribute you created earlier to the value received from the service.