The stock is down because the general market is down.
What are people even investing for? For like 4-5 months? If you're serious about invested it should be on the basis of the company's long term fundamentals not the ups and downs of stock price. There is no stock in the world that will rise upwards all the time even if they hit every sales mark they want to hit.
Every tech company is down, Sony is way down, Apple is way down, Microsoft is down, Samsung is down.
Like always, Nintendo's stock has taken a hit due to a number of reasons.
Analysts thought Nintendo was being too bullish on the Switch's shipment forecasts, mobile hasn't taken off as much as was initially hoped between Mario Run, Animal Crossing: Pocket Camp, and even Dragalia Lost, most deemed Nintendo's software release calendar light compounding the issue of the shipment forecasts, and then, of course, the overall market is down significantly in the last 6 months.
Mobile might still be an issue, but Mario Kart could very well be the game to crack the market fully for them. The shipment forecasts will most likely be close or exceeded. Then they also have a very strong position moving forward with lots of options to expand the Switch's market reach even beyond what it currently has. I'm bullish on the future of their stock and think it is seriously undervalued by the market right now.