Onscreen spies are supposed to be so good that they can fool the other characters in the film and the viewers watching at home. Usually, the best spies rely on their superior wit to dupe an enemy, but occasionally, a spy might use a costume or two to trick an opposing force. Most often, though, it's subpar spies who rely on silly fictional disguises. That's why they always get caught. Ridiculous wigs and bad prosthetics are the favored weapons of dumb, undercover criminals who flee the law.
Some terrible disguises are supposed to look farcical, though. Dana Carvey's ludicrous get-ups in 2002's Master of Disguise were created to be mocked. They parody the "man of a thousand faces" trope in spy fiction. Characters like the ones below, however, tried their hardest to fool the masses.
Leonard Nimoy's Japanese Impersonation In 'Mission Impossible' Is Hard On The Eyes