Was the Mexican-American war a positive or
negative aspect of Manifest Destiny?

The war lasted 2 years they lost almost half of their territory. Which lead to manifest destiny encompassing land from the Atlantic Ocean to the Pacific. Most people thought it was a negative thing.

nothing certain is known about what happened, but it seems that the country declined gradually, perhaps due to bad government, or, more likely, due to a series of bad harvests, which resulted in widespread and recurrent famine.