If you think stupid people are stupid, you are judgmental. If you think ugly people are ugly, you are judgmental.
It has adopted a negative connotation as the practice of paying any unbiased attention to behavior pattern of others becomes taboo. It's only socially acceptable to acknowledge all successes and instantly forget all failures, that way no true judgement of a person's behavior pattern can be formed. If people weren't afraid to actually tell when others were incompetent, much of modern society would unfold.
A: <says something really really stupid>
B: <makes WTF-face>
A: "OMG I hate you- you so judgmental."
C: "Yeah wtf, A, learn some manners."