I think keeping up your appearance is a good thing, but it shouldn't be how you identify and value yourself. You can make your appearance a god just like anything else. If we learn to wrap our identity up in Christ, only then will we be content.When we give God our best, even in the area of our looks and dress, that is enough. Along with that I think we need to be careful in how we judge others by their looks.
Though I don't use the version of Bible they use in this article, it has some good references and content that I hope will encourage you.
1 Samuel 16:7 says "...the LORD seeth not as man seeth; for man looketh on the outward appearance, but the LORD looketh on the heart."
Proverbs 31:21-22 "She is not afraid of the snow for her household: for all her household are clothed with scarlet. She maketh herself coverings of tapestry; her clothing is silk and purple."
If you are dealing with any sort of low self image, I want you to know that you are not alone. I've caught myself comparing my looks and dress to others. It's easy to get caught up in that. But we must remember who it is we're trying to please....the world or God? No, I don't dress great all the time. Obviously I don't wear my "best clothes" to work outside and sometimes I get caught out in public wearing those grungy garments. Sometimes I wake up tired and the outfit I put together really wasn't that great after all or maybe it's that I didn't have time to do my make-up. It happens. In those moments, there is a choice to make...feel self-conscious or be confident in the person God made you, knowing that's not how you always present yourself.
I'm not an expert, I'm just here sharing some opinions and things I've learned. I hope you feel encouraged. Have a blessed day.