If something has decreased significantly or too much, you can say that it has "dropped too low" or "fallen too far." For example:

The stock market has dropped too low in recent weeks.
The temperature has fallen too far below freezing.

Alternatively, you can say that something has "fallen too much" or "decpned too sharply." For example:

The value of the company's shares has fallen too much since the start of the year.
The number of tourists visiting the city has decpned too sharply since the outbreak of the pandemic.