Two options:
1. Johnson humiliates himself, and Clinton benefits from the collapse in the third party vote
2. Johnson overperforms, due to a shift in perceptions. The increase in third party vote throws the election out of whack, and alters the outcome in close states. Example:
I think that Johnson's appearance could feasibly benefit either candidate, both of these are just random hypotheticals. I really don't think Johnson can win an electoral vote in 2016, but he could definitely flip a few states between the Dems and GOP