Hi -
I would think it is certainly unusual, but probably not a serious 'problem' - I wouldn't think it means her eyes are 'faulty'; in fact we DO see things 'upside down' on the retina at the back of the eye, I believe, and the brain converts the image to be the correct way up.
Does she ALWAYS do it upside down, even when she is on her own for example? I guess it's possible she does it because you are opposite her, and she's drawing so that YOU can see it the right way up.
I was twenty years a TA in primary schools, and I do remember a child who could do writing upside down so the person opposite could read it.
Does she go to pre-school/nursery yet, and if so what do staff there make of it? I think young children see 'words' as shapes in the early days of learning to read, hence they confuse 'was' and 'saw'.
Do you know if she sees equally well with each eye? You could possibly check that yourself, covering one eye at a time with a clean piece of card, and asking if she notices any difference. Some children have eyes that 'flicker' from side to side, and I think that can be hereditary.
But if you are able to, it might just be worth having her checked by her GP, HV, or an optician. Better to be safe than sorry.