To me, what's 'inappropriate' depends on the situation. An outfit that would be in place at the club might seem very inappropriate to wear to work.
I just think there's a time and place for wearing more revealing clothes. And this comes from someone who loves wearing clothes that may be deemed 'inappropriate'. I only dress like that when I'm going out at night, not really any other time.
So yeah, if you mean just day-to-day dressing, clothes that are too revealing (they either don't fit right or they leave nothing to the imagination) generally seem inappropriate. I personally only think of someone as a 'slut' if I know them personally and have seen them do slutty things, but many other people are not so forgiving.
Nothing wrong with revealing outfits, its where she decides to wear them. If its at work where every guy all day is going to be looking, she becomes one o f those girls that's nice to look at for the day, but far from interested in her because that's not my type, a girl who is OK with every guy checking her out that way all day, and not just the guy she's marking up at night.
I don't think a girl is a slut for wearing revealing clothes, but I don''t think its necessary to wear such short dresses, that when you are standing up I can see your underwear :S, or shorts so short, that your bare ass is hanging out.
I think 'too' revealing' would be deemed as innappropriate. A *little* revealing is fine, but having your boobs hanging out, or wearing see-through clothes isn't so modest. I don't mind it, and I'm sure some guys won't either but you will be secretly judged and attract the wrong kind of attention.
Outfits that reveal a little too much are inappropriate, and does tend to give others a bad impression if they don't know you. Boobs popping out or booty shorts that don't cover anything at all - usually gives off that "I'll do you in a back ally" vibe. You can still wear clothes that appropriately cover everything, and still look attractive and sexy.