Here's a direct comparison between a Cardinal (Cardinalis cardinalis) and a Mocker. The first three notes are the Cardinal, the second three are the Mocker imitating a Cardinal. The Mocker's version is nearly perfect!
A Northern Mockingbird (Mimus polyglottos) marks his Summer territory in the mesquite near Peck's Lake [Arizona] 0. His scientific name means "Many-Throated Imitator" - and that he is! In this 10-second sample, he sings 8 distinct songs, that's an average of a song every second and a half or so. The entire recording lasts more than 90 seconds, and he never repeats! Many of the snippets he sings are easily recognizable as the Cardinal, Cactus Wren, and various orioles.