Why is the stratosphere warmest at the top

Temperatures raise as the altitude increases. It also absorbs all the suns rays and energy making it warmer. 

A small research in google :

"Temperatures rise with increasing altitude in the stratosphere. Ozone molecules in the stratosphere absorb ultraviolet radiation coming from the Sun. The energy from the UV radiation is transformed into heat. The heating is most intense near the top of the stratosphere, so that is where the stratosphere is warmest."

In short, the stratosphere is warmest at the top because it absorbs UV radiation and also more the altitude is high more it's warm.

Hope this helps !

Photon

Sequoia national park  /  inyo national forest, california, u.s. mount whitney is the tallest mountain in california, as well as the highest summit in the contiguous united states and thesierra nevada—with an elevation of 14,505 feet (4,421 m).