I've heard Mayor Quimby mention the people of Charlotte, NC before also.
He did. It's on the episode "Bart-Mangled Banner" back around Nov. 2004. It's where Bart accidently mooned the US flag behind him in school.
Mayor Quimby compares the national hatred of Springfield caused by Bart's mooning of the U.S. flag to that of "Hitler City, North Carolina" before it changed its name to Charlotte.