Extremely overqualified character does poorly on a standardized exam or competition
Saitama (OPM) - Strongest character in the verse broke every single physical record, but failed the written portion of the Hero Exam, resulting in him being placed in Rank C.
Richard Reed (Marvel) - "Smartest Man Alive" has, in multiple continuities, failed his driving exam.
Charlie Chaplin (real life) - allegedly lost a Charlie Chaplin Lookalike Contest
Dolly Parton also lost a Dolly Parton lookalike contest, but in her case it was a drag contest, so...