1. 'Well-known dressed' doesn't mean anything at all - it makes no sense. 2. 'Posh clothes' are nice clothes, but it isn't necessarily negative. If you say to your friend 'You look posh tonight' you're simply remarking on the fact that she has 'dressed up', and it could be a compliment. 3. 'Dressed up' isn't necessarily negative, but you could say that some-one is 'too dressed up'. This implies that the way they are dressed is inappropriate for the occasion. If someone turns up for a picnic wearing a designer suit and gold jewellery, they are 'too dressed up'. 4. You could also say 'over-dressed'. If you go to a party in an evening dress and high heels and find that everyone else is in jeans, you would say that you feel 'over-dressed'.