All of the DLC that was released on the PS3 and Xbox 360 is now available on the Wii U eShop.  There is a rather large update that has also been released along with the DLC.  It is kind of odd that this content was released without any formal announcement from the developer or publisher.

Free Skins
Flashpoint Batman
Luchador Bane
Containment Suit Doomsday
Classic Ares

Characters ($4.99 each)

Skin Packs ($2.99 each)
Bad Girls (Catwoman, Wonder Woman, Harley)
Teen Titans (Raven, Cyborg, Deathstroke)
Flashpoint (Aquaman, Deathstroke, Wonder Woman)
Killing Joke (Joker, Joker, Joker)
Red Son #1 (Superman, Grundy, Wonder Woman)
Red Son #2 (Batman, Green Lantern, Deathstroke)
Blackest Night #1 (Superman, Flash, Doomsday)
Blackest Night #2 (Batman, Hawkgirl, Aquaman)
Arkham City (Batman, Catwoman, Joker)
New 52 (Superman, Batman, Wonder Woman)
Earth-2 (Flash, Hawkgirl, Grundy)

Single Skins ($0.99)
Man of Steel Superman