Actually, your mileage only varies when it comes to your personal reaction. In-story, it's not only outright stated, but even becomes a minor plot-point: she is not only cruel and evil, but inhumanly proud of her actions, thereby ignoring Granny's Mind Rape.
Fridge Logic: Some of the ramifactions of the witches moving Lancre in time. The people don't generally seem too dismayed, but fans have wondered about Esk, the Lancre-born female wizard from Equal Rites. This may possibly be alluded to in I Shall Wear Midnight - but possibly not.
It also crops up in later books - shouldn't Granny Weatherwax now be another fifteen years younger than her childhood acquaintances like her sister Lily (Witches Abroad) or Mustrum Ridcully (Lords and Ladies), not to mention when she goes to visit Mrs Palm again in Maskerade (having previously met her in Equal Rites)? At this point one really just has to resort to the MST3K Mantra.
Mustrum Ridcully commented that she hadn't changed a bit, so maybe appearing 15 years younger than expected helped.
This is actually one of the reasons Terry eventually wrote Thief of Time, to explain away the wibbly-wobbly, timey-wimey nature of Discworld chronology. It all makes a twisted sort of sense that the progression of time is inconsistent between places.
Fridge Brilliance: It's fairly obvious that the plot is based on that of Macbeth, but only those who know the background of the play (or read about it later) know that Macbeth is also like that: in Real Life, the historical Macbeth is usually considered to be the good guy and Shakespeare wrote the play in order to appeal to King James VI and I, who was the descendant of King Duncan.
Genius Bonus: The Felmets' plan (based on the Fool's ideas) is to have Hwel write a play that presents them as the good guys and King Verence and the witches as villains, altering public perceptions. This is exactly what the original play Macbeth was, with Shakespeare whitewashing his patron King James I and VI's ancestors and painting Macbeth as a blackguard, when the historical record suggests he was seen as rather a decent king.