Depending on what type of riding you do on your mountain bike it may make perfect sense - shorter cranks increase ground clearance reducing the likelihood of rock strikes etc. possibly allowing you to more easily pedal through a technical section. This could be more important to you than the reduction in ability to make torque quickly that will come with the shorter cranks. If the majority of your riding is downhill, with shuttles or pedalling back uphill done on a fireroad there would be little downside to shorter cranks. That said, there are other circumstances where cranks that short would be horrible on a mountain bike because it's very hard to spin.