Would it be so wrong to tell someone they're a slob?

If you wish to tell this woman the truth, then I think you should. She is owed an honest reason for your reservation of her getting in on the car pool, and it will make you feel better as well. It is ok to tell her that, while getting in on the car pool is fine, it is the condition of your vehicle that has rubbed you the wrong way...and then explain in a bit more detail. It is only courteous to keep a tidy vehicle when offering [expected] rides to others. If she pitches a fit, well, she will get over it or be mad a long time. perhaps she will clean out her car and you will all ride happily ever after.