When you first ask that question, before anything else, ask why you are asking it. Is it to find how much is too much of an emphasis on physical appearance? Or is to find out if Christians should care about physical appearance at all?
The answer can be broken into two parts. First, remember this: you are an amazing creation who was made by the Creator of the universe, and every time you wake up in the morning, you are a testimony to the creativity of our God. Psalm 139:13 tells us how God wove you in your mother's womb. Nothing man can ever create now or in times past can even come close to God's creations. So in that sense, yes, you should care about your physical appearance as far as nurturing and caring for the body that God has given you.
Which brings us to the second part: remember that God judges the inward heart, not the outward appearance. Your heart is where your true beauty lies. As amazing as the shell that contains all that makes you you is, it pales in comparison to the beauty of your inner self (1 Peter 3:3-5).
While it is important to stay physically healthy, your outward appearance should not be the sole focus of your attention and energy. So how do you keep the focus off of your outward appearance? That is really a matter of constantly checking your heart attitude.
One question to ask yourself might be, "Am I dressing a certain way or trying to get to a certain weight in order to impress others?" If so, then it is time to re-evaluate your motives. Matthew 23:12 says, "For whoever exalts himself will be humbled, and whoever humbles himself will be exalted." (Also see James 4:6.) Your value is not determined by what you look like externally; this is a lie the world will constantly try to feed you. When you begin to think this way, recognize it for what it is: a lie (Romans 12:2).
Then what are we supposed to do? We shouldn't let ourselves go completely because that's unhealthy. But we shouldn't focus on the way we look all the time because then it becomes a point of pride or even shame. How do we find the balance between the inside and the outside?
Do take care of yourself physically, but guard your heart against letting emphasis on your outward appearance become more important than growing in your relationship with God. Remember, you are a marvelous, beautiful child of God, and if you are a child of God, why would you need to impress anyone else but Him?
God judges the inward heart, not the outward appearance. Your heart is where your true beauty lies (1 Peter 3:3-5). While it is important to stay physically healthy, your outward appearance should not be the sole focus of your attention and energy. Check your heart attitude and motivations for altering or improving your physical appearance to make sure that they are humbly healthy and not attention-seeking (Matthew 23:12).
Heidi Joelle spends her days staring at paperwork and making sure it is where it is supposed to be, how it is supposed to be, when it is supposed to be. And then she comes home and makes sure the porky little dog isn't eating a trashcan. Between these two events, she tries to learn and see as much of the world around her as possible.