The Rotom possessing a stereo in Pokémon Pokopia is called DJ Rotom.
DJ Rotom is a named character whose Rotom has possessed a stereo system. The nickname connects Rotom's ability to inhabit electrical appliances with the character's music-oriented presentation.
Rotom is especially associated with changing forms by possessing devices, so a stereo is a natural fit for the Pokémon. Pokopia uses that familiar trait in a social and character-driven setting rather than treating it only as a battle mechanic.
The name can be mistaken for a generic description such as “Stereo Rotom,” but the character's canonical nickname is DJ Rotom. It also differs from other specially named Pokopia characters, whose names reference painting, cooking, tools, or unusual physical conditions.