Yeah, you know what, I've always wondered this myself. What exactly did he do that was evil? All he did was tell the truth

I don’t think the mirror was evil. The mirror simply gives an opinion but even if the magic mirror was always right. You can’t argue with facts so the queen is either delusional or she is very insecure.

IIRC, he has enough villainous depictions in other pieces of Disney media (or just generally treated as a villain) that he's allowed to count.

fact, towards the end of the film, it's subtly implied that King Magnifico ends up becoming the Magic Mirror of Snow White and the Seven Dwarvesdue to the Mirror's face making a quick cameo appearance as Magnifico is sealed inside his staff's glass shard. If it were confirmed that Magnifico did become the Mirror and went on to serve Queen Grimhilde voluntarily, it would make him even more viler due to driving Grimhilde into villainy, essentially making him the overarching antagonist of that film and his pe status would get affect as he would have gained redeeming features and became anti villain