I imagine it's because while she is very likable, some people think her character occasionally stagnates in her development. As in we'll get multiple epsiodes where she comes out of her social anxiety, only to find her back in it next time. Being honest though, I never understood this complaint. Social Anxiety can be complex, and just because she is doing better in one area (Fear of performing in front of others, from Filli Vanilli), doesn't mean she doesn't still need work in another (Fear of Nightmare Night, from Scare Master).