Okay, I understood my mistake. It's because of the "der" which says it's singular. If I wanted to say "are they the mayors?" I would say "Sind sie die Bürgermeister?" since the die says it's plural. But it could be "Are you the mayor(feminine)?" either.
"Ist sie der Bürgermeister?" would be "Is she the mayor?" The capitalized "Sie" means it is the formal second person.