In the anime, at least, it seems that the girls are much more conscious about showing skin or cleavage than in other anime set in modern times (except for the beach episode).
Especially when you take into account the target audience, it is rather odd.
Do the light novel or other official sources delve deeper into this cultural element of the fictional world of Mahouka?
It is indeed mentioned in the light novel.
(All the excerpt from the light novel quoted onwards are from Baka-Tsuki)
From Volume 2 - Chapter 7, during the scene where Tatsuya was called to the Counseling Room by Ono Haruka (emphasis mine):
[...] Haruka's smile grew a little forced as she crossed her legs.
Beneath the mini-skirt, the derriere wrapped in silk stockings could almost be seen.
[...]
According to modern decorum, it was advisable to keep revealed flesh to a minimum.
Because women usually wore thick stockings that covered everything beneath their dresses, even when exuding maturity, this sight was quite stimulating to the eyes. [...]
Speaking of which, she was also wearing a rather open shirt, where the lines of the undergarments could be seen.
For a faculty member to wear this in front of a student was overly flirtatious.
From Volume 3 - Chapter 1, after the ball game in physical education class, when Tatsuya and Mikihiko made acquaintance with each other and Erika came by to chat:
In 2095 AD, fashion and proper decorum demanded that any trace of exposed flesh be covered in public. Since school was considered a public environment, even if it was in summer, women were still obligated to wear jackets and a colorless, impenetrable pair of tights or leggings beneath their dresses.
That being said, these rules didn't apply to athletic uniforms, so athletic clubs suffered no penalty or criticism for revealing their hands or legs with their uniform, and physical education classes were also not bound by this rule. For example, Tatsuya and the other male students were currently wearing shorts that did not extend to their knees while Mizuki was wearing a fairly long pair of rhythm pants, which was the standard uniform for physical education.
On the other hand, Erika was......
Both of her legs were completely uncovered.
From the thighs down, everything was revealed to the sweltering summer air. Her pants had no length to speak of whatsoever. Finally, the T-shirt she wore barely extended over the pants, giving the impression she was wearing only her underwear.
It is also again mentioned in Volume 3 - Chapter 4, during the scene where Mayumi turned up late in her summer dress and asked Tatsuya of his thought about her appearance.
The characters' consciousness towards the dress code is also shown indirectly through Hattori and Mayumi's expression in the scene slightly after when they are on the bus, where Hattori offered Mayumi a blanket to cover herself up.
And the issue is brought up again a bit later into the same chapter when Tatsuya and Miyuki arrived at the hotel, where they met Erika and Mizuki in "revealing" outfit (according to the standard in Mahouka's world).
In addition to nhahtdh's answer, I would also like to add that Volume 8 - Chapter 2 of the light novel also talked about temperature dropping (a mini ice age) in 2030 AD, and the temperature only went back to normal recently. Then, the light novel went on to attribute the drop in temperature to the changes in the fashion/dress code.
—Beginning from around Common Era 2030, the earth underwent a radical drop in temperature; the world’s overall food supply deteriorated on a massive scale.
[...]
Looking at that vivid greenery and sparkling ocean, makes the world’s temperature drop seem like an event in a fictional story.
Nevertheless, that event is an unequivocal fact.
Before we were born, the world’s climate began getting warmer but, we can look at the traces of the temperature dropping all around us.
For example, the dress code.
Not exposing bare skin, this manner in which we dress is nothing but a trace of the deep impression the era of the temperature dropping left on the world.
Well, I have no interest in dressing to expose my shoulders or breast area — and in the first place, it would not be a flattering style on me, yet—, it’s not compulsory to have a skirt so long the hem drags, and I like clothes. However in private places, there are no rules over the manner of dressing so it’s not like it really hinders us.
