The Jane's Addiction member who appeared in the 2003 film Uptown Girls was Dave Navarro.
Navarro appears during the film's celebrity-heavy funeral sequence, where he is credited as a "Rock Star." The scene also includes other music figures, including Mark McGrath, Nas, Carmen Electra, and Duncan Sheik. That ensemble of recognizable guests can make the cameo easy to overlook, especially because Navarro is not central to the plot.
Dave Navarro is best known as the guitarist of Jane's Addiction, the influential Los Angeles alternative-rock band formed in the 1980s. He also played with the Red Hot Chili Peppers in the 1990s and has worked extensively as a solo musician and television personality. Strictly speaking, Uptown Girls credits his role as "Rock Star" rather than using his real name as a character, but the question's intended identification is unambiguous: Navarro is the Jane's Addiction band member in the movie.