Mickeyjunk Mountain is the Wasteland version of the Matterhorn from Disneyland. It is the location where Oswald the Lucky Rabbit puts every piece of Mickey Mouse-related merchandise that ends up in Wasteland. As the name suggests, it sounds like "Mickey's junk", as Oswald refers to it when the mouse first meets him.
According to the graphic novel, the mountain is located in a remote part of Wasteland, where only Oswald ventures, and it has been created by Oswald himself by secretly ammassing every Mickey Mouse object brought there: he did so because he did not want Mickey's visage to be seen around what he considers his own kingdom.
Oswald resides in his own private sanctuary atop the uppermost peaks, stewing in a constant reminder of his "brother's" endless success. Also on the Mountain is the jug in which the Shadow Blot has been sealed. Oswald stays on the mountain to guard that jug and make sure that the Blot doesn't escape.
In Epic Mickey 2: The Power of Two, the players can no longer access Mickeyjunk Mountain. Due to the damage caused by the quake, the mountain is deemed too dangerous to visit. Instead, the mountain is replaced by the Dahl Engineering Corridors.