I think part of the problem with both Cadence and Twilight has been that a lot of fans are still clinging to the notion developed back in season 1 that alicorns are near all-powerful gods in spite of everything the show has done to disprove that idea since then. Lauren Faust commenting on DeviantART a while back that if it were up to her there would be only two alicorns in the series didn't help either.

As far as Celestia goes though portraying her as an evil dictator is almost always done as a joke, not many fans actually have anything against her.