Personally, if anyone said any of this stuff to me, I’d flip.
It’s not (necessarily and IMHO) me being completely resistant to criticism- the point is that there are a lot of variables.
If this is something so blatantly obviously a violation of tznius that the person just doesn’t seem to care, then MAYBE I would get the rebbetzin or someone with some kind of official capacity (NOT a random do-gooder from around the block) to broach the question. A lot would depend on whether the person would be willing to listen- it’s not a mitzvah to rebuke someone who doesn’t want to be told, IIRC, and it can actually make the matter worse.
If the person is wearing something that in other communities is acceptable but in this one is not (eg. a knee covering skirt in a place where people usually wear mid-calf, a more bright but still tznius outfit in a more somber environment) then as long as there are no actual issurim being broken I think this is absolutely counterproductive. If you want, just keep the whole “holier-than-thou” mantra running through your head and whenever you see her, just remember that you’re better than that poor, deluded, pritzusdikke soul.
(Yes, I’m bitter, thanks for asking. Though the actual advice is 100% serious.)
I go to a pretty yeshivish shul where I am one of the more, shall we say, creative dressers. Nobody cares, which is nice. I have, however, been in similar shuls where some nice lady told me that my skirt is too short or my top is too bright. I don’t get dressed blind, and my mom doesn’t buy me clothing that she doesn’t think is tznius. If I’m wearing it, that means that my family and therefore my rav thinks it’s fine. (The most annoying is when someone tells me my shoes are too “trendy”- I mean, honestly, in three months you’ll be wearing them, and there’s nothing intrinsically WRONG with them tzniuswise. It just doesn’t make sense.)