I'm not so certain this behavior is common. I'm sure there are just as many men looking for dates from the women who come to train in aikido. And if that is all they are interested then I don't think they last very long. But I've seen most people of both genders focused on their training while on the mat. If you find it common where you are then the sensei is not setting the proper tone in the dojo.
I find this kind of behavior, whether male or female, inappropriate. Someone should suggest to this woman to cover up somehow--whether it's a bra or just a tee-shirt or tank top (if she likes being free). It's not about puritanism or modesty but about creating a positive training atmosphere for both men and women.