Of course it's inappropriate. Would you, as an adult woman, go to visit your gramps and granny with your tits and arse out? I know I definitely wouldn't, it's disgraceful.
What's the purpose of clothing like that? I've seen a girl of about 13 last autumn. It was quite chilly, windy and rainy. She was wearing those tiny lycra up-your-butt shorts and a bralette with AF1s. Essentially she was wearing underwear and not even particularly covering ones.
For what? Yea, it's not 'for comfort', no one will convince me that shorts wedged high up your ass is comfier than a lose dress, trackies, whatever. She was shivering, it was cold, hardly comfy. What for then? To show her body off? To whom? Why does a child need to show half her ass hanging out? Where's her mother?
I was with my 12 year old and to us she looked pitiful. Like that child prostitute from 'Taxi driver', exploited and misguided. But there will be plenty of men of all ages oggling her and thinking something else entirely.
Why would any parent allow this, I'm not sure. Why would you want to look sexy when going to see your elderly grandparents, I'm even less sure. My parents wouldn't allow me out of the house dressed like a whore (as they'd put it) and now that I'm an adult, I entirely agree with them and their terminology.
Coincidentally, it's entirely British thing. Young girls made up and dressed like prostitutes. Won't see that anywhere else in Europe.