First of all, I haven't seen Frozen yet, so this answer is more or less based on common sense rather than any critical assessment of the movie itself. But you have to consider that Frozen was not nominated (let alone awarded) in categories like Best Picture or Best Screenplay, but in the categories Best Animated Feature and Best Original Song.
So while the story or other details might not have been on "Oscar level" (I don't know, though), it is the overall composition of the movie that counts for Best Animated Feature. Add to this that in this regard it wasn't competing with American Hustle, 12 Years a Slave or Gravity, but with other animated feature films of that year. So it is not a big stretch to be regarded as the best of all those animated movies, seeing that it got pretty high commercial and critical acclaim in general (other factors aside, like being released in late autumn and thus favouring the Academy's sometimes short attention span).
Likewise, while I am not able to judge the quality of the song "Let It Go" compared to the other songs nominated for Best Original Song (though, I would probably have chosen "Ordinary Love", but that is entirely based on personal preference), this is again a quite specialized category, where the success of this movie is not too far-fetched or based on other things like its story.