There is nothing wrong with being concerned about fashion in the gym. otherwise there would not be such a huge selection of gym attire out there to be bought. There are people who don't wish to look like their mother's didn't teach them how to dress themselves before they go out of the house.
Nike did not get to be so huge with just their shoes.
I stick with Nike and Addidas, which can be bought anywhere.
I usually go with sleeveless tops made from some type of dry-wick material, and typical mid-thigh length shorts. Finish it off with running shoes and short white socks. Simple and clean.
Why bother with fashion, "they" ask? Some people just care about their apperance, that's all.