Yes, you should be attracted to a woman you are thinking about marrying. But there is a lot more than hair in a relationship. If it is a real worry for you, talk with her about growing out her hair. See if you are attracted to her with long hair. If you do not like thin hair, and that is what hers turns out to be, then you need to consider carefully. Remember, when hair is the only determining factor in a relationship, it will not likely last. Your wife needs to be a partner, companion and friend for a mariae to work out in the long run.
My own wife had a collar length bob when we met, she grew it out to hip length and has kept it below the shoulders for our entire marriage ( 38 years). Sometimes longer, sometimes shorter. She knows her hair is important to me and we reach agreement where its length will be from time to time.
She also knows that an amazing head of long hair is going to turn my head every time. She is not threatened by that interest, of course, she knows I would never act carnally on that interest.