This is going to be a very off-topic post. :o :)
The robe is part of the uniform that sufficently covers the body. I don't think there is a need to specify the inner garments especially given that the general wizarding wear are robes itself.
Originally Posted by minnabird
-At some point something was said along that lines that a lot of modern wizard kids wore Muggle clothes during holidays.
They would be awful because they probably don't keep up with what is in fashion in the Muggle world. Wizards are occasionally shown to be backwards in many walks of life compared to the Muggles. Perhaps what they try is to ape the current trend so as to not look out of place. Trends change.
-If wizards regularly wore Muggle clothes under their robes, why would they be so awful at wearing them?
And the wizarding alternative of a woman's nightgown would be? Like Azhure said, it's probably about what Pureblood/Half-blood/or what different age groups prefer.
-Old Archie at the Quidditch World Cup makes it fairly clear that he's used to wearing something similar to the Muggle nightgown he's wearing at that time...(I do not mean to imply that wizards do not wear underwear. I'm pretty sure they do.)
The illustration in DH at the beginning of the chapter 'Kreacher's Tale' shows the photograph of the Marauders Harry finds in Sirius' room. In the drawing, the four students are clearly wearing some sort of clothes beneath their robes. James and Remus have button-down shirts while Peter and Sirius have t-shirts. In the chapter 'A Place To Hide' I think Ron and Harry also have clothing under their robes when they are shown hurrying along Tottenham Court Road. I assume illustrations are canon.
-When Snape is flipped upside down, we see his underwear. While it's possible he's just a weirdo, given the rest, I'd say this could count as evidence.
It appears to Harry that he had indeed taken the opportunity to take his clothes off and replace them with school robes. But this is Snape we are talking about. See that this is actually the fifth time that Jo makes a reference to Snape's clothes in five pages. He could easily be an exception to this general rule because he's probably never wore decent Muggle clothes, which therefore explains his evasion to them. This could also explain his wearing just pants in SWM.
-Also, when we see his memories in DH, during the part on the Hogwarts Express it mentions something about him looking more comfortable wearing his robes rather than his uncomfortable, ill-fitting Muggle clothes. If he were still wearing said clothes, would the robe really help his uncomfortableness?