I personally don't think you should ask her out yet. You said she's nice, but average. I think if you're going to have a relationship with her that will last, you have to know in your heart that she is the best girl in the world for you. I think it would definitely be worth it to be friends with her, amd you may start liking her back, but I think you two should get to be close first.
Don't waste her time and/or hurt her if you're not interested, because that's what will happen. You are either interested or you aren't, period. If you're not, move on. Just because she's nice, you don't owe her anything and vice versa. Only pursue people you honestly want to be with.