There have been some interesting points brought up. Firstly, I don't think weight has anything at all to do with dressing nicely. I do think self confidence is an issue for some people and weight can be a part of that, but I have seen stylish and well dressed people at every weight and income level. I also don't think dressing down has anything to do with hygiene and that this is a completely separate issue.
Just to be clear, I also think that appropriate dress means dressing for the situation. Most situations these days allow for a more casual appearance and so there is nothing out of place in wearing casual clothing. Times have changed and as eating out became more commonplace it also became less of an event and the dress requirements became less formal. There are situations that do call for business casual, business, semi-formal and formal attire, though. It is in these situations that dressing down stands out like a sore thumb.
I compare dressing appropriately to having good manners. Being considerate of others sometimes takes slightly more time or effort. There are no laws concerning manners, just as there are none governing what you wear. You are free to blow your nose on your shirt sleeve, spit on the street, scratch your backside, chew with your mouth open, and not hold the door open for people behind you. No one will arrest you for doing these things, but you will be judged by those around you. Same with how you dress. There may not be a law governing it, but there are tangible benefits to dressing appropriately.
There are countless studies showing the effects of personal appearance and its far reaching consequences. So while it may not be "right" or "fair" that as humans we judge by first impressions, that's just the way it is. Someone who presents themselves well will be treated better in hundreds of tiny little ways, many of which will be done unconsciously by the people around you.
As a final point, I know I always personally feel a little sorry for the women I see who are clearly spending so much take taking care of everyone and everything else around them that they don't take care of themselves. As a teacher I've seen this quite a bit. Moms (I mostly seem to meet the moms) who are clearly wonderful and caring parents who could look so much better. I don't think less of them, but I wonder when the last time they felt beautiful and desirable was. These ladies always make me wish I could treat them to a spa day and a makeover.