Matt Groening has confirmed that Michael Jackson did provide the voice (under the name John Jay Smith) for Leon Kompowsky. He did not do any singing which was done by an impersonator. Jackson wanted an impersonator to sing because he wanted to fool his brothers into thinking the impersonator was him. Jackson did record himself singing but the tracks were not used.
Michael Jackson called Matt Groening and requested to voice a character on the show. He was a big fan of Bart and promised to write a number one song for him. The song he wrote, "Do the Bartman", reached number one in many European countries in 1991.
Michael Jackson performed the speaking voice of Leon Kompowsky under the pseudonym John Jay Smith. Although he was able to lend his voice to the speaking parts of a character, the record company would not allow him to sing. As an alternative Jackson entrusted Kipp Lennon, a founding member of the folk band Venice, with the job of the singing parts of the character. Lennon has contributed to several episodes of the series.
This was the second time that a major celebrity guest appeared on the series. The first was Dustin Hoffman who appeared in The Simpsons: Lisa's Substitute (1991) credited as "Sam Etic". Following Jackson's use of a pseudonym, the producers no longer allowed celebrities to use fake names.