After being around since the beginning of what you might call a crusade for justice, Karlach's overall ending portrayal has been more than satisfactory.
I believe it actually does her character more justice than any gleeful or happy ever after ending, such as tending bar or doing mercenary work occasionally while living the simple life back in Baldur's Gate, visiting her parent's resting place now and again.
It sticks close to what the writers intended for her, her story arc still evokes that emotional response everyone was so fond over, bittersweet or not. Yet it has closure mixed with an open ended, never ending adventure, which is what Karlach's character is all about. Seems contradictory to have both closure and never ending story conclusion defining her ending, but I can't seem to explain it any better. It just works.
TLDR: Karlach got the ending she deserved. It is not the one she "wanted" but it is the one she "needed". It is also what the fans needed as well, they just didn't/don't know it, I think.
It has enough meat to push another game even, with the spine and bulk of the story being about Avernus. Pretty epic ending for a character to be able to promote such a foundation and be an integral component to making the story relate to the events directly preceding BG3.
