I want Discourse to work more like Mastodon?

Titles are crucial to a topic. I don’t see a reason those should be hidden or removed. If your community members can’t write reasonable title, they need to get a lesson on two about how to use a forum. If you want to get more fancy, you can invest time in building a plugin which sends a random title for a topic and then hides the title area through CSS … but that will make your entire site very, very very weird with all the hidden titles.

If you just fancy the social media look, try the fakeboook theme