Mirror writing is actually pretty common at that age. If she is still doing it in a couple of years look at her for something, but right now no worries.
I'd say reversals on individual letters are common, but not complete sentences. Do you think she really doesn't see it as backward? I enjoyed mirror writing & writing upside down as a child. Denying that it was atypical sounds exactly like something I'd say, even though I clearly knew what I was doing. If she REALLY can't see the difference, & this persist for the next few months, I'd look into it more.
I actually still write upside down for my job on a regular basis. So maybe my teachers were wrong & it wasn't wasted time!
