Curls can be perceived as unprofessional, however, a company cannot discriminate based on whether someone's hair is curly or straight. That means in hiring, and in making comments on someone's personal appearance after someone has gotten a job.
Likewise, a woman with curly hair cannot be made to wear her hair straightened just because the look is perceived to be more professional than curly hair, and she cannot be made to wear her hair up, or tied back. A person - male or female - can be asked to maintain a clean, professional, groomed appearance, and that is all. For men that refers to facial hair and the length of their hair (usually if they have long hair, it must be neatly cut, and sometimes tied back), for women that usually refers to tidiness of their hair. Frizz is a natural hair property, however, so a company can't say to a woman: your hair is frizzy, we don't allow that, do something about it. Just like they can't say, dye your grey roots, we don't allow you to have grey roots, it's not professional.
In general, hair has to be washed, not greasy, and not uncombed. Technically it doesn't even have to be styled; a woman can just tie it back, or she can trim it herself so that it looks neat (easier for a woman with straight hair to do, I think).
Now, the nicer your hair looks, and the nicer you dress, the better your overall professional image is, and the better kinds of jobs you might be able to get, or the better your chances of promotion, etc. But that relates to overall image, and IMO overall image is just a part of the whole, of which ability is a larger part. I personally don't subscribe to the faction that immediately leaps to the assumption that there's curl discrimination in the workplace, but that's because I've never experienced it. I've never had a boss or a coworker ever make a negative comment on my curls in over 20 years. Not saying it doesn't happen, of course. Just that I haven't had it happen to me.