Nobody ever says this about mediocre white women, or any POC of any gender
Seems to me that these people want to date people of high social status/privilege in society - and due to mediocre white men’s complete lack of any highly positive qualities - by virtue of being “mediocre” - can more easily be stripped back to only being viewed as white and a man as the sole standing “positive qualities”, making them a more pure symbol of status/privilege
“Mediocre white man” is kind of an overpowered class. You get huge bonuses to like everything, and you can put your points pretty much anywhere. Like, all checks against law enforcement are like roll all dice thrice keep best. Just for free. You don’t even need to actively join the faction, but if you do the bonuses stack.
More seriously, I’ve never heard anyone seeking out a mediocre white man on purpose.