It's absolutely normal for a teenager to dwell on his appearance - that topic and sex are often the two most important things on his mind. When my son was going through this he would spend an hour with things like showering, fixing this and that, dealing with clothes and his hair, and asking his sister how he looked, and at the end of it all he would shuffle out of the house muttering "I look a right fucking mess" or something like that.
So I think the bottom line is that at this stage in your life your appearance means a great deal to you and there's nothing wrong with honoring that and seeking to look good. You aren't being shallow; you're being you.
Nobody living can ever stop me
As I go walking my freedom highway.
Nobody living can make me turn back:
This land was made for you and me. (Woody Guthrie)