To those who equate loshon hora with lack of tznius:
Loshon hora is usually not pre-meditated; lack of tznius usually is.
When we label someone “frum”, we mean that s/he truly tries to live in accordance with the Torah.
When a woman wears clothing that are fine according to the letter of (some Rabbonim’s interpretation of) the law, but are not in the spirit of the law (make the statement, “I wish I didn’t have to keep these laws”), she demonstrates her lack of desire for frumkeit. (I agree that this is usually due to emotional issues- e.g. low self-esteem, lack of true domestic harmony- and not because she doesn’t believe in Hashem.)
That may be why some people refer to wearers of barely-covering-the-knee skirts as “not frum.”