We got most of our inklings from seeing the 1990 film The Nutcracker Prince. Although it inevitably uses Tchaikovsky's score, it seems to be the most faithful adaptation out there of E.T.A. Hoffman's "The Nutcracker and the Mouse King".
There's a feeling of post-Secret of NIMH animated audacity in the film, because there are onscreen deaths and actual blood, which is probably why we remember scenes from it so vividly. In particular, I recall the flashback in which we see the princess become ugly, where everyone is drawn in a more stylized '60s-ish way for some reason. The Mouse Queen is voiced by Phyllis Diller (Ha ha HAAA) of all people, which makes things even stranger.
And despite the Mouse King sounding remarkably like Tom "Spongebob" Kenny (it isn't), he is by far the most gosh darn scary Mouse King ever. Watch the climax and you'll see what I mean.