This is rather reminiscent of Paul Merson dismissing our new signings by saying "I've never heard of them." There are plenty of things I've never heard of but that doesn't mean they should automatically be discounted.
And it was precisely that attitude that allowed him to become Labour leader and then to wipe out the Tories' majority. I think the time to underestimate Corbyn has passed. It's not Corbyn who is the muppet ...