Miranda had a role that was notionally supposed to be a substantial, important part. She's the only ME2 character to have a recurring appearance throughout the game, hinting at something big and then finally appearing at the end. The problem was the context, the focus, and the sheer lack of information about what she was doing.
It focused too much on her hunt for her sister.
Many of us Miranda fans had a lot of issues with this, as it was seen as a rehash of her loyalty mission to an extent. Granted, she's one of the few people who goes off to solve her own problems instead of relying entirely on Shepard. But as I said, they wasted a perfect opportunity for her character to instead focus on something that was resolved. They focused on her emotional aspects and made her out to be someone who just wanted to be loved and to be normal, someone the PC would have to pity. They really could have (and should have focused imo) on her being a badass super-spy whose problems have been taken care of and is out working to see what Cerberus' plans are.
She could have been investigating Cerberus and Sanctuary, finding out her father was involved, and later, finding out that her sister had been one of the refugees that had taken shelter there. We could have had her support for the Sanctuary mission, and later have her support for Cronos if she were to survive. As well, they could have had her give more substantial updates and leads on her progress, giving useful information and showing that she was being the hero of another story that I believe BW was trying to do with her (while dropping the ball heavily.
Hell, her story could have even been the plot for a side-game or comic, explaining what she was doing in ME3 and how she was doing it. But the focus on her emotional side and the cinematography involving her physical assets (there, I said it), as well as several developer statements about her character (most prominently Hudson) make me feel that they had no other use for her character than what they thought up. It's astounding, since she's actually a strong female character that isn't too masculine or off-putting. She's strong while maintaining femininity, even if they did focus on making her overly sexual.
All this said, her story was a lot better than what it was supposedly in the original script: You wouldn't see her or hear from her at all until randomly finding her as a prisoner at Sanctuary, where she had a forced death. I don't know if its true or not, but we Miranda fans had been hearing all about it for a while.
Ask people like ieldra and taboo. Granted, I think they've been gone for a while.