1. A crouchwalking decoy moves much, much faster than an actual crouchwalking player, making that decoy very unconvincing.
2. There's no way to do a walking decoy. Just running decoys, sliding decoys, and crouchwalking decoys. Given how humorously robotic Mirage's walking animation is, this seems like an oversight. Having more decoy behaviors available would give us more ways to mix up our decoys to make them convincing.
I mean lets be brutally honest here, mirage as he currently stands is basically a rookie killer and holds very little functionality against anybody who knows what to look for.
He looks like a cool legend on paper but lets face it, his ultimate is his only redeeming feature as you can make some pretty nice flank plays with it.