@Paralleluniverse I think you are misunderstanding her character big time because all you thought she was about was being "badass".
She always loved Eren. Her motivations all revolved around Eren. It's always been that way. Her enlisting into the military? Eren (or to be more specific, so she could protect him in case the need arises but it was always about staying by his side. Heck she was willing to give up on that If he couldn't make it). Her fighting nail and tooth against titans?? All Eren.
She, unlike Eren, never really changed. What you say was "lost" wasn't really: Mikasa was pretty calm and collected... Until it involved Eren. Then she would get emotional and irrational. We had so many instances of this on the first arcs, I'm surprised you can even call her "strict and logical". Is no wonder she would be so rattled when the center of her affections starts to do some seriously dangerous shit, to the point people around her are starting to wonder if they should kill him for endangering their island. Add to that Eren was purposely leaving them in the dark so they could muster up the courage to kill him and end the eldian curse.
Anyways, Mikasa never got a lot of substance. Sorry to say, but she is always been like this and she finally gets some closure or conclusion to her arc (by having to let go of the person she loves, that is destroying the world). Sure, maybe she could had some more going for her but If Isayama never bothered then is probably because that was all she was meant to be: A girl in love in a cruel world.