Text on Featured ads

I do not know what do you mean?

I mean, if you feature an ad, is the a functionality to add some text on that advert to show some text like featured or paid advert on it

The featured ads have CSS class advert-is-featured you can use it to add some content using “content” property https://css-tricks.com/css-content/

How to customize specific elements you can find out here https://wpadverts.com/documentation/featured-ads/ in the Customization part.