The full title, given in the last chapter, is:
THE DOWNFALL OF THE LORD OF THE RINGS AND THE RETURN OF THE KING
(as seen by the Little People; being the memoirs of Bilbo and Frodo of the Shire, supplemented by the accounts of their friends and the learning of the Wise.)
Together with extracts from Books of Lore translated by Bilbo in Rivendell.
There's nothing really strange about it; snarkily, one might say that it's as if The mysterious case of Ellen Ripley was actually called Alien, or as if Sarah Connor and what she got up to next was actually called Terminator; in other words, it is actually quite common, and should not be seen as surprising, for a work to be named after the antagonist.