The rock stars are three hand puppets who sing in a fictional band.
The rock stars inspire Oobi, Uma and Kako to form their own band. They never appear in person but are seen on Oobi's TV. They are a trio of singers who are likely based on the Beatles, due to the similar sound of their music and their mop-top haircuts. The lead singer (on the right) has a deep voice, while the other two have much higher voices.
Since they're only shown briefly, only a small snippet of their "lyrics" can be heard:
Each rock star has fair skin and long, brown hair. Two of them wear dark sunglasses, and the other has hair so long that it covers where his eyes would be.