It all started when Finnwyn became popular and then eugenics became a part of it and reached meme status.
When Finntress fans objected to it people brought up that HW isn't looking for a long-term mate but was still interested in "exceptional beasts" like Finn.
Then someone pointed out that HW had a cameo in the episode "Wheels" where Finn was playing the flute. Now we know that Finn is too tired to care and is being forced to play it by Jake, but to the outside perspective (HW) it looks like Finn is trying to impress the only girl in the vicinity who was also around his age. This is after Flute Spell where she kissed him so one would expect her to step in, but she didn't. She watched him flute it up for a bit before sneaking away, thus creating the HW cuckquean meme.
And honestly, it kind of fits her character.
She believes in the survival of the fittest, so she only cares about the alpha (Finn) creating strong babies.
She is afraid of commitment where as Finn wants to settle down.
She has a lot of stealthy spells to aid her voyeurism .
Although calling her a cuckquean is disingenuous since she isn't raising anyone's kids and she will have her own exceptional babies with him, but she definitely watches Finn go at it, sometimes in the closet, sometimes from the window, always wearing a Superman costume.