It depends on why she's mixing them up.
My daughter does NOT have dyslexia. For a long time she would forget which way to write numbers, did it go left to right or right to left? Because she messed it up so often, it created anxiety. The anxiety made it difficult for her to figure out which way to write her numbers. She would actually forget.
For us, going over why numbers are written the way they are seemed to help a lot. I know they cover it in first grade when they are first learning about number places, but not all schools continue to remind the kids about it. Going over number placement (ones, tens, hundreds, etc.) and reminding kids that numbers aren't just an abstract symbol, seems to solve a lot of the issue. For example, writing 52 apples means that there are 5 baskets of 10 apples and one basket of 2. I had to remind my daughter of that well into 3rd grade.
I really do think your first step is really getting her to understand numbers themselves. Once you've determined that she really does understand number placement and she is still mixing up her numbers, then it might be time to see if shes dyslexic.
Good luck!