I don't mind her ending, it's fine. It's the end, not every end needs to be happy. Sad endings works just as well. Arthur Morgan died horribly and that was a better ending than any of the ones this game offers. I rather mind that everything you do up to that point is just so... empty. Find a random thing, bring it to the Smith, get some exposition.... Hardly the stuff that the not-last-minute Origin characters get. Say what you will about Wyll, at least there's things to do with him. Even last minute fan service Halsin has a better quest and more to do than Karlach.
This game is the first in this genre in a very long time that completely refrains from annoying collectibles and quest structures like; 'Collect items 1/5' and yet, Karlach's is exactly that.
The fact that she's the obvious stereotypical lesbian interest has nothing to do with this. In fact. The game panders to every special interest group and yet, every such group has someone complaining there needs to be more for theirs. If it were any other character with such an ending, someone else from another group would've had the exact same complaint.
I'm currently surprised there hasn't been a complaint about vegetarian auto-select options for camping.