I'd say not authentic although it's not a bad attempt at a mid to late 70's signature. The first part of the signature looks pretty good but the final half doesn't seem right.
Well, it's a great item to have autographed although it must have been pain unfolding it outside of the "Dakota Hotel" to get it signed.
I'd put it in the same category as the John signature on that "Greatest Beatles Signed Photograph" in that I would have said no myself but I'm happy to defer to the experts.