It's a pretty sensical and predictable movement though; Nintendo's first real foray into smartphone games becomes the most successful launch ever, leading the stock market to rationally believe that Nintendo will begin to really push their IP into this space and make lots of money. The irrational party is Nintendo, since they now give every indication that the success of Pokemon Go will have no effect on their strategy; the stock market has now responded and said 'we don't think Nintendo will make as much money in future as we'd previously predicted'.
In fact, you're making the same mistake investors did: Believing Nintendo made a foray into smartphone games. Nintendo did not make Pokémon Go, nor is it receiving much profit from it. Niantic, Google, and Apple are all getting the lion's share of the profit, Pokémon Company gets a bit of it too, but then Nintendo only gets a fraction of that.
I don't know if it's irrational. If you, or anyone else can be more rational, and predict the market's errors, you could make huge amounts of money.
Predicting errors is hard though. You could have shorted Nintendo's stock after Pokemon go came out. But what if they started announcing more Pokemon games for mobile, and they were also huge successes?
> If you, or anyone else can be more rational, and predict the market's errors
You can't rationally predict irrational errors. At best you could have spotted the trend early and jumped off at the right time, but that is still speculating on the degree of irrationality.
If you can't predict them except in hindsight, well then how can you say it's irrational? If you can't tell me whether or not investing in Nintendo stocks has negative expected value, you shouldn't call people doing it irrational.
Of course there was. If a company produces the most successful mobile game of all time, and shows there is a huge interest in one of their IPs, you don't think their stock should be worth more?
Hype is often the prospect of a huge success, but it always remains a long shot. As another comment said, you can short the stock each time there's a hype according to you and you'll often make a profit--until you don't, and you'll take a very heavy loss.
There are a serious amount of people users, many from a new audience. It is the first breakthrough "Augmented Reality" application, paving the way for a new type of game that everyone can enjoy.