As a concept: Something that is trying to hard to be cutting edge and hip and cool but is about as edgy and cool as a warm bowl of oatmeal. "alt" for the masses.
"I am the same, yet I still think I'm different, in a uniquely special kind of way...I'm Mallternative!!...hey, look, it's a Journey's right next to a Hot Topic!!!"
Betty: "I found them at this little store over on 11th street. There's a lot of cool, funky shops there. A real mallternative to where I usually shop."
Wilma: "A mallternative? What the hell is that?"
Betty: "Think about it dumbass."