Normally, her outfits tend towards the sort of parent-approved clothing that indicates that Mom and Dad have a firm idea of what appropriate attire for a teenage girl is.
We've seen her in a rather demure dress:
rocking the Annie Hall look:
starring as an extra in a "Blend In With The Crowd" event at an ad for the Gap:
taking fashion tips from Velma Dinkley about how cable knit sweaters flatter the figure:
as well as getting in tune with Sailor Moon and dressing like she's a Japanese schoolgirl:
while her go-to summer ensembles are either "T-shirt and Bermuda shorts" or "sundress".
The reason that I mention this is that if an outfit a character wears is not one someone associates with that person, there's generally a good reason for it. Since Martha impresses me as being a normal person from a normal family, the only conclusion that I can derive is that her parents are not aware that she left the house dressed like that and would be rather perturbed that she did. I can imagine dark commentary this summer about tops that show too much belly and how that gives that makes that skeevy little git she wants to pretend is Mister Wonderful think all of the wrong things.
I can also imagine that Elly assumes that they'd actually let her wear something like that because she sees herself as being the only parent who cares about what her children do. It doesn't matter that this in practice means blocking them from her mind when she's not looking at them in the belief that when she stops paying attention to them, they just freeze in place, she convinces herself that she and she alone haas standards.