Clearheaded:
As you pointed out, Yiras Shomayim & Yiras Chet go hand in hand. The problem with Tznius is either:
1: The women in question are lacking in Yiras Shomayim in general (which begs the question why are they lax in Tznius over other Mitzvos).
2: The women in question have Yiras Shomayim, but don’t believe they are doing a Chet.
Both are possible, and both probably account for many of the problems we see.
“Lacking Yiras Shomayim in a certain area” does not make sense (to me). Either they fear G-d or they don’t, but not that they only fear G-d for certain things. Punishment maybe, but not Hashem.