I'm beginning to wonder if the people who like Minthara are also the same people whose first Star Trek was Voyager.
Yeah I am interested to know who exactly she appeals to. I'll never follow the evil path so I watched the romance scene on youtube. To me it was pretty meh -- some soft porn with some mild D/s elements. Liara's review said the D/s elements weren't 'filmed' correctly so it doesn't seem that her audience isn't lovers of kink. My guess is that her character specifically appeals to those who appreciate sex as reward fantasies.
The narrative tension in ST: TNG was "you have a set of principles, can you keep to them in the face of credible challenge"? Which, as you note, was worn down in subsequent Star Trek variations. My personal theory for the popularity of the Witcher type "grey morality" / "no alignment" is the U.S.' endless wars. Soldiers are fighting a war that started before they were born and no one seems to know what the greater principle is -- outside of some generalized sense of patriotism or the like. So a character like Geralt that just does the job they were given and who has a limited ability to influence the larger events taking place in the world resonates in a way that character that CHARNAME with their ability to bend the universe through the application of willpower does not.