In 1-syllabled adjectives, we use -er in the end for comparative degree. Cool is a 1-syllabled adjective. So, we use cooler when comparing to another thing. e.g. the weather is cooler than yesterday.
However in certain situations, we use more with cool. e.g. She is more cool, patient and sweet than her friend. Here we have three adjectives, so more cool looks appropriate rather than cooler.
In 1-syllabled adjectives, we use -er in the end for comparative degree. Cool is a 1-syllabled adjective. So, we use cooler when comparing to another thing. e.g. the weather is cooler than yesterday.
However in certain situations, we use more with cool. e.g. She is more cool, patient and sweet than her friend. Here we have three adjectives, so more cool looks appropriate rather than cooler.
Refer to this page to understand the basics of degrees.