I personally think its less because it makes you look 'more Jewish' (I am myself and I don't think I look more Jewish with curly rather than straight) and more just not liking curly hair. There've been a bunch of threads over time where people just don't like curly hair. Maybe it's because it isn't 'expected' and they don't like that, or because it's more 'wild', whatever, I think it has more to do with that than religon/culture/whatever.