I'm starting to think that there is virtually no correlation in Tumblr posts' popularity beyond the random people who see it and happen to reblog it, thereby spreading it for more people to see.
My picture of FeMC dressed as Nina has 13 notes. Not much.
My picture of Chie dressed as Jack Frost has 26 notes.
My picture of FeMC in a Yasogami uniform has 93 notes.
All the pictures are similar in quality, with maybe the Yasogami one being slightly worse.
*shrug*
I don't get it.
I think you need a combination of luck, timing and money to get popular on tumblr.
It's a damn shame too, since I genuinely think that your art is as good / better then most of the popular art blogs I follow. If only there was a way to get our PersonaGAF artists on the map...