Our family subscribes to the idea "You only get one chance to make a first impression". You never know what the day will bring or who you'll meet along the way, so why not choose an outfit that looks put together? It takes the same time to grab a well fitting, stain free outfit (even less time to toss on a cute sundress) than the former mentioned frumpy look. If you don't have time or the inclination for make-up at least wear a smile, it's the best accessory you have!
I do feel that one's sense of self worth is often perceived by others from their first impression of you (right or wrong as it may be). Sometimes in a young childs mind, when they feel that their mother doesn't care about themselves, they may correlate that into how could they have the ability to care about me? Yes it may just be cloths to some or the overall feeling of self worth to others, but I feel that the way we treat ourselves shows the world how we want to be percieved and treated in turn. Shallow maybe, but honest.
All this from an extrovert if it makes a difference.