I installed the WP User Frontend plugin to see what it is doing. The easiest way to get nice looking posts published to Discourse is to publish an excerpt from the post. You will probably have to create the excerpts manually before you publish the post to Discourse.
To make this work, make sure the WP Discourse ‘Use Full Post Content’ option is not selected. Before you publish a post, add an excerpt to the Excerpt meta-box that is on the WordPress post-edit page. (You may need to enable the excerpt meta-box in the WordPress Screen Options menu. The excerpt should be published to Discourse. On Discourse, when someone clicks the Show Full Post button, they should see the properly formatted post.
There are other ways that this could be approached, but they all require some custom code.