Miranda may have been a missed opportunity but at least there were still some redeeming features (romance, control chip convo, email, Sanctuary) and the rehashed bits (Oriana focus), while mundane and tiresome didn't break the character.
Kasumi on the other hand:
-was the only squadmate to just outright refuse to join you. Not "I'm busy doing something else" or "I could be of better use elsewhere" just straight up "I don't want to." Even Zaeed the amoral mercenary with the same DLC status as her had better reasons and reactions.
-never mind romance potential, the aforementioned refusal coupled with the way the lines were delivered makes it sound like she actually resents Shepard for ME2. It's the opposite of Liara. Her you can't get rid of. Kasumi you can't be friends with.
-the damnable graybox. ~2 years later and she still can't get over her dead boyfriend to the point where if you kept it she would rather spend her last days plugged into it as opposed to doing anything useful. And even if you destroy it the epilogue slide is still her alone staring at it. Admittedly what she's holding doesn't look like the graybox we saw in ME2 but the tone of the image is anything but moving on. The story is literally unwinnable for her.
-the synthesis slide. I joke about wanting to vomit at the thought of synthesis. That slide actually makes me reach for the Pepto Bismol.
Even in Citadel, the fanservice DLC, every character gets some nice quality time with Shepard and at the party. Kasumi on the other hand seems incapable of being a real person and is reduced to pop-up and underwear fetish jokes. I'm probably one of the most politically incorrect people I know but damn, that's racist. They might as well have had Jacob eating fried chicken.
So no, I don't think Miranda got it worst. She was woefully underused, wasted really. Her character had zero growth and was diminished from ME2. But at least it was still intact on paper. Kasumi got absolutely butchered.