From what I remember, the Mirror on the Wall never said she was ugly, just that Snow White was "the fairest of them all," or "the prettiest/most beautiful one of them all." She could have been pretty still, just not the prettiest.
Also, one of the moral points the fairy tale is making, is the fact that the queen is ugly inside.
The mirror also states that she is the most beautiful "here" but that Snow White "beyond the seven mountains" is "a thousand times more beautiful". Really rubbing it in, that mirror.
That story is at least 120 years older than Disney's implementation. In the original the huntsman brought back Snow White's supposed lungs and liver which the queen then ate.