Looks play a large part in leadership as much as one would try to deny it. It's shown repeatedly in hiring preferences, political races, etc. For example, look at the relevance of height in presidential races.

Charisma, looks, and finally intelligence seems to be the importance that society places on leadership roles. If it's a democratic society it will end up this way.

If it's a noble society and not just strength based then the leaders will desire those that have high attributes in all three sections. Their children as such should slowly grow more desirable unless they inbreed to protect wealth/claim/possible power.

Anime that feature a demon lord ruling by sheer power will fall into two categories. Like the noble society though, assuming they are capable of producing children and are humanoid it seems likely they would want peerless partners. If they additionally use physical violence they will also be strong/fit/attractive - this is common in some male demon lord MC's as well.

If they are non-humanoid/undead the question isn't relevant as they are likely using sorcery and terror to role. We see less female MC's of this type, possibly due to the women-are-wonderful effect and not wanting to desecrate the female form?

So the question turns into a different question - what type of society are the ruling over?

Oh, and lastly people generally don't want to see the grotesque so aesthetic taste of the audience comes into play. Most "demon" based anime are hardly dark - many have demons has the main protagonist too or allies (such as an isekai featuring a demon kind race).