We don’t have a written policy, but we do have some guidelines. In general, we want the main f-droid.org repo to be something you can show to kids, grandparents, collegues, etc. We do not want to be the morality police. F-Droid puts no content restrictions on other repos.
Something like WallHaven seems totally fine to me, as long as the app entry in f-droid.org (description, screenshots, etc) are SFW.