Is hottest a comparative adjective

No it is a superlative adjective, it ends in -est. Comparative adjectives end in -er.

comparative - bigger, smaller, lighter, tidier, tastier.

For adjectives with 2,3,4 syllables use more not -er eg more exciting more interesting, more flexible.

Superlatives - highest, biggest, tidiest, tastiest.

For adjectives with 2,3,4 syllables use most not -est eg most interesting, most exciting, most flexible.