>> Is their clothing somehow inherently offensive, or do you just dislike it because it doesn't fit your own aesthetics, and/or because you associate it with a negative type of person?
The latter, the association. My aesthetics are questionable in various ways all by themselves. But associations are a different thing.
People pre-judge each other on clothing all the time, and people very often broadcast who they are and their group affiliations through clothing. If a pub is full of people in football shirts I'm probably not going in because it's not my sort of place. I have no desire to be around loud sports appreciation. It's not because I think they're beneath me but I know what to expect.
The question of class and chavs is an interesting one to some extent - I don't judge them because they are poor, working class people. It's the behaviour, the general poor education etc. I don't think it's wrong to judge this as bad, nor to associate particular behaviours and outlooks with clothing when there is an associated look - I'm an old goth, we definitely had our look and our commonalities.
Let's turn this around - a woman goes to the supermarket in her food-covered pyjamas, with a kid in tow, screaming at them to 'shuh uh!'. You're not going to make any judgement on that?
How's about a massive guy with long hair in a metal t-shirt and a denim waistcoat? No preconceptions?