A quote you can use

From here. 

Being well-dressed is a beautiful form of politeness.

Now of course, different people mean different things by this, but hopefully we can all agree on the principle.  If my outward appearance is (within reason) pleasant and unoffensive, and does not unduly distract others, that is a sign of good manners--and a needlessly unpleasant, distracting, or offensive outward appearance is in turn a sign of bad manners.