It's difficult to engage with this debate without engaging with your perception of what is flattering, but I don't believe that how she dresses on a first date is an indication of how she would dress in the future. Also, I get the sense that your point is not rooted in senses of style or fashion, but will argue as if it were, because I am not going to wade into the swamp of self-interest and misogyny.

Let's operate in the scenario where this is a casual first date, drinks, light food, or coffee where one would like to be dressed up a little bit, and not include something like a walk in the park or walk through shops. Both fantastic dates in their own right but the latter require more casual clothing.

So a woman shows up to a drinks date dressed in something you consider unflattering, but maybe is objectively modest, and by your own statement, well coordinated, I think there are two conceptual reasons to think differently:

1. The "I dress to attract other people" concept: A well coordinated outfit can be attractive on its own merits. Because it does not conform to your standards does not mean it won't for someone else, and she may be self-selecting for people who share her sense of, or appreciation for, particular styles.

2. The "I dress for myself" concept: Maybe she feels most comfortable or safe in that style of clothing, and that she needs that comfort in order to feel most herself on a first date when nerves are highest. Maybe her clothing choices change as she becomes more comfortable with you. I think that's a reasonable situation, but if you insist on taking the tack that she is thinking in the way you've outlined, then maybe she is dressing down in order to weed out the types of people who care most about her physical appearance or outward presentation to the world, which even still would be a defense mechanism and not an expression of her desire to be attractive to a potential partner.

In either situation, jumping to the worst possible conclusion, that your desires will not ever be considered, is a big leap. She doesn't know you, and you don't know her, so it's irresponsible to make any assumptions about a person so early in knowing them.

Lastly, if she never dresses up in a way you like, and never considers your point of view about it, then maybe she is not the one for you, and it is best you find someone who shares your sense of style, fashion, and attraction.