One might point out that the light jazz-inspired piano soundtrack is both soothing and catchy, and that such originality surely deserves to be called epic...but while the soundtrack is very much both of those, it would not carry much weight in and of itself. It’s perfectly tailored for the game, and - in this writer’s opinion - is better enjoyed in that medium.
One may finally inquire as to whether or not two-player co-op pushes the game over the edge and into the abyss of epic...alas, the co-op is unfortunately fashioned in the same vein as the game itself: as fun as a trampoline in a swimming pool, but without real purpose.
Kirby’s Epic Yarn seems to exist just because it can. For example: in single player the difficulty is so ridiculously low that I have never once been harmed by an enemy, and the few times that I’ve fallen into a pit I was minimally penalized by losing some of the gems that I had collected. 100% completion is the real challenge of Epic Yarn. Co-op is treated exactly the same way. While the clipping issues from New Super Mario Bros Wii are long gone (players can move across each other freely), there is still some player interaction: you can carry each other and toss each other. Once that wears off, though, the straightforward approach that makes single player content easy makes the co-op almost...boring.