sort of, hes a giant plannet biult sa pamamagitan ng a monkey (thats G1 for you) but the monkey lot control of unicron. but unicron has a weakness, the matrix of leadership. Rodimus prime destroyed unicron and now his just a head floating in space.(more info) he destroys plannets and moons for energy as he continually destoys them.
Allow me to answer your tanong too the best of my ability. According to trasnpormer lore, before the dawn of time, Order and Chaos existed within an extra-dimensional entity known as The One. To explore the fledgling universe, he created the astral being known as Unicron, and then subdivided him, creating his twin, Primus.
Unicron is the eternal arch-enemy of his twin brother Primus. Also known as the Lord of Chaos, the Chaos Bringer, and the Planet Eater, he is dedicated to consuming the multiverse. His massive form is powered sa pamamagitan ng the consumption of planets, moons, stars, and even the very fabric of existence.
So, no Unicron is not a Decepticons sa pamamagitan ng any means. He's his own entity who's sole desire is destroy everything in existence, including the Decepticons.