With no IoW, does she really have a reason for modesty besides the ESRB? Not saying you should make her nude in her classic skin, but lore-wise I don't see why she'd wear a fur bikini when she's surrounded by wild animals who don't care if she's clothed or not.
Note that I'm not trying to be totally serious here. It's just one of those funny thoughts you get.
With no IoW, does she really have a reason for modesty besides the ESRB?
That is literally the reason. The governing rules of the culture that creates the art still influences how the art is created. Art itself is just a lens we can see through, one that is distinctly colored.
I would assume the bra is to prevent her breasts from flying around everywhere being annoying and stuff. The lower thing, I cannot explain.
Stylistic choice, mainly.
She also wears boots, jewelry and warpaint, wields a spear, and has flowing lustrous hair that's obviously cleaned and well cared for.
None of those things have a reason to exist on her if she is a true savage, and never learned about human culture or technology.
if you want to see her naked around wild animals, you should give rule 34 a try, i'm sure i could fish up something from the dank abyss if you like.
Wait, did you think 
's pants were Summoner-enforced?
Clothes still offer some form of protection even just a little bit.
Also I might be part of her transformation like its not clothes but an extension of her body.
There could be a lot of different reason no one knows for sure not even Riot :P.
On the one hand the clothes might offer some protection(I doubt that clothing was invented out of modesty) and I doubt that Nidalee never had any contact to other humans considering that she is able to speak without problems.
