A collection of series set in a world where beauty standards are the opposite of ours. So ugly equals beautiful and vice versa. Like in 'The Twilight Zone's episode 'Eye of the Beholder'.

In most cases the protagonist gets Isekai'd to this world and builds a harem of "ugly" girls. The heroines are often discriminated against for their looks and thus have have low self confidence or in some cases mental health issues. So there's a wholesome/white-knight aspect.

[Created this list as there's no tag for it. These are not recommendations.]