I don't think it's low effort, it's rather efficient. From my point of view, the problem with Karlach is basically what makes her popular, she is that perfect cinnamon roll from the start. She has not much of a character arc because there is no need for her to have one. You can't even corrupt her because she is too decent for that. She has not much to offer apart from being heart warming or heart wrenching. I think she is wonderfully written and acted the way she is, but there is only so much you can do with a character that does not have any internal conflict.
Really, my only big issue with her ending currently is, that on certain characters, you cannot stop her from exploding in the end, despite going with Wyll being her default decision.
But she does have internal conflict. She's torn apart by her desire to live and her fear of Avernus. This comes to a head during her post-Gortash-fight rant and leads into your descent to Avernus. As much as she talks about wanting to go out on her terms, as much as she says she'd rather die than go back to hell, deep down she regrets not being able to live out a full life. She tells you as much in that rant.
It's just that there aren't enough moments like this to more smoothly transition her into accepting a return to Avernus as a choice that she makes for herself. This way, her character arc goes something like this:
An unwilling soldier desperate for freedom hitches a ride on a passing ship away from the battlefield. But freedom is never free, and she soon realizes that her time is limited. She's destined to die unless she goes back. The soldier frantically looks for a solution to her terminal problem. While there is some hope in the beginning, failure after failure forces her to come to terms with her eventual fate.
Refusing to go back to the battlefield, she decides to take revenge on the man who sold her out and then die on her own terms. But when she strikes him down, after the adrenaline rush recedes, she realizes that this changed nothing. She laments how empty her revenge felt and how unfair it was that her life was stolen from her. She admits to you and to herself that, after all, she does want to live. Yet, at that point, you've done all you can to no avail.
Time is running out. After defeating the big bad, she has one last chance to choose between facing her fears or dying then and there. Realizing that fate wouldn't allow her to simply run away from her problems, she chooses to face them head-on for her own sake. She goes back to the battlefield but this time she's not alone. She finally has people who care for her and who believe her life is worth living.
There's your character arc right there. Throughout her story, she gains the conviction to face her deepest fears. That arc exists right now, they just need to flesh it out more.
As it stands, going back to Avernus is a choice that we make for her at the last minute. Mind you, that choice was always there since launch day, it just didn't have a cinematic and, for some reason, Wyll was the one to go with her regardless if you romanced her.
