According to Google search engine reports, Harley Quinn is the most popular Halloween costume for this year.
Joker's sidekick and sometimes-lover has taken the number one spot, closely followed by Star Wars, Superhero, Pirate and (all-time classic) Batman. (source)
With the upcoming release of Suicide Squad, it doesn't surprise many analysts due to promotional artwork and material being released... slowly.
Her presence in the latest, installment of the Arkham games may also attribute to the new found popularity of the character.