The lyrics from the Panic! At the Disco song read "So testosterone boys and Harlequin girls/Will you dance to this beat and hold a lover close?" It serves as the chorus to the song, repeated several times throughout and forming a key part of the back beat. The remainder of the song discusses a break-up and the singer's bitterness at being left for another man.
A Harlequin was a type of Renaissance clown, which first appeared in Italy in the 1400s. Harlequins were a part of commedia dell'arte festivals, a form of improvised theater that stressed pratfalls and social satire. Harlequins specifically were very dexterous and agile; they usually wore masks and tight clothes featuring a diamond pattern.
A Harlequin's agility constitutes the first part of a "Harlequin girl's" definition. While she likely doesn't 't tumble or perform cartwheels, she certainly demonstrates physical skills, which the music lyrics state take place while dancing. Thus, a Harlequin girl is someone who knows how to dance--and by possible inference can perform equally well sexually.
The Harlequin's mask and make-up lends another aspect to the term "Harlequin girl." In the most obvious sense, it means a girl who is flashy and draws attention to herself through her dress and appearance. In more subtle terms, it can mean a girl who hides her true intentions behind a mask, presenting an alluring or mysterious facade in order to entice onlookers.
Like other clowns, a Harlequin also appears as a fool or a buffoon. In terms of the phrase "Harlequin girl," it may have several meanings. When taken in context with "testosterone boys," it may refer to girls who are foolish enough to let hormone-fueled boys get close to them. When looked at in terms of the song as a whole, it may refer to one particular "Harlequin girl's" foolishness at leaving the singer for another man.