No, she's not based on a real person. If she was a real person, she wouldn't be single- and chances are I wouldn't be either, and I probably wouldn't still be wading through shoulder-high piles of clutter, myself.
And if she was real, and was still single, chances are she'd be single for a reason- the popular internet phrase back in the day was "don't stick it in the crazy".
Also, it depends on your opinion on what constitutes a "mess". I know people to whom a single wadded tissue paper on the coffee table or two dirty cups sitting in the sink is a "shambles". Then again, I also know people whose walls are stained a deep yellow with nicotine, there's a literal 3" thick mat of dog hair under the couch, and the kitchen windows are nearly opaque with grease from all the fried foods, who would tell you "this IS clean. You should have seen it before I straightened up."
We know from Cara's description that her bed was dirty and likely unmade, there were dirty dishes in the sink, and the trash hadn't been taken out recently. The rest of the "mess" might have simply been some DVD cases on the coffee table, a few of the books not put away, and a couple of takeout coffee cups that hadn't been tossed yet.