Pinkie’s probably one of the few who don’t know who Judge Fear is, thus his face of fear would have no effect because she wouldn’t be scared. Considering the only canon people it never worked on was Judge Dreds (self explanatory, he’s just that badass) and some kid who didn’t know he should be scared
“In Gotham City, the fearsomely evil Judge Death is restored and clad in a semblance of his former uniform. After killing the Scarecrow’s henchman, he attempts to turn on the Scarecrow himself only to be stopped by a dose of Fear-o-sol which exposes him to his greatest fears: fluffy bunnies, My Little Ponies and other cutesy characters.” http://en.wikipedia.org/wiki/Batman/Judge_Dredd:_Judgment_on_Gotham
http://en.wikipedia.org/wiki/Batman/Judge_Dredd:_Judgment_on_Gotham