According to some measures, Tom Petty is more popular. For example, he has sold more albums than John Mellencamp, and his songs have been streamed more often on Spotify. Additionally, Petty has won more awards, including multiple Grammy Awards and induction into the Rock and Roll Hall of Fame.
On the other hand, some measures suggest that John Mellencamp is more popular. For instance, he has had more Top 40 hits than Petty, and his music videos have been viewed more times on YouTube. Mellencamp has also been more successful in the international market, with several of his albums reaching the top of the charts in countries outside the United States.
Ultimately, it is difficult to say definitively who is more popular between Tom Petty and John Mellencamp. Both artists have enjoyed successful careers and have made significant contributions to American rock music.