That's a possibility, but I think she just messed up. I think she took off both plates when she got near Travis's neighborhood, so no one would be able to trace her when she arrived/or left his house. She probably planned to calmly drive out of town and put just the back plate on (because you don't need the front one in AZ). Then, after the killing went way beyond what she planned, she freaked out and only remembered to put the back plate on in the desert when she was getting rid of the evidence. Either by doing it too quickly or because it was pitch black outside, I think she just slapped it on upside down.
But the alibi theory is a good one too!
But that's why she had the rental car so the neighbors wouldn't recognize her
car, or her blond hair.