I'm mixed and really I don't see what her being mixed has to do with anything. There really is no such thing as "mixed" hair. That's a HUGE myth that way to many people buy into unfortunately.
Whether her mom knew how to do it or not is really irrelevant to her actual NATURAL hair type. I think it's been a LONG time since Alicia Keys has had her natural curl pattern as it grows out of her head. Her hair has been damaged from dye and heat..clearly. She's hardly ever seen in public with unmanipulated hair. And one can see from her childhood photos her curls aren't the same today. And I still think she was texturized in her senior photo..a lot of mixed girls used to texturize their hair (and still do frankly) back in the day..they just didn't tell anyone and pretend it's natural. And it wouldn't surprise me if she got a fresh texturizer for her senior photo.