I can't remember when I saw my first example of sexy Velma cosplay. It was probably over 15 years ago, before we had Instagram and before cosplay was a common thing. At the time, it was probably quite surprising, that a minor character from an old cartoon was not only being cosplayed, but being cosplayed in a sexy fashion. At the time, the idea was probably to be surprising and subversive: Daphne was the designated cute girl, while Velma was the nerd in dowdy clothing. But for some reason, the idea of sexy Velma Dinkley caught on, and now, a dozen years later, it is a common cosplay. What started out as a subversion has become a cliche.
Not that I am objecting too much. The idea of our shy sidekick getting her chance in the limelight makes sense as an appealing dream to a lot of us, and even while it is titillating, it manages to be wholesome. Sexy Velma cosplay is just one of the things the internet's unusual pattern of emergent behavior has brought about, and will probably be seeing it for a while.