Here's some reading as to why this may be harmful --
http://en.wikipedia.org/wiki/Stereotype_threat
This is very different from dissuading girls from participating because of how they express themselves or how they behave. Offering makeovers is, instead, a way of preying upon girls' insecurities and suggesting that, despite whatever success they may have had on their team and at the event, they will be better off still if they conform to some societal ideal of beauty. There is a marked difference between targeting girls that already have an interest in makeup and targeting girls with makeup and this seems very much like that latter.
Edited to add: I don't necessarily care that a cosmetics company is sponsoring FIRST, really, but do take issue with offering makeovers during the event. A better, more meaningful approach might be to promote FIRST at cosmetics-related events rather than the other way around. All of that being said, having read a bit more about Ms. Tilton today, I am not really comfortable with her being anywhere near FIRST at all. I would not offer her as a role model for the students on our team.