Do people really find brunettes unattractive?

Not at all. Preference to hair colour is a matter of everyone's personal taste. One person might find blonde to look really silly and brunette to look very tempting and attractive, the other might find blonde cute and brunette boring. Another one might not like anything but ginger. The tastes vary as much as hair colours do, and for every hair colour there are people who find it very attractive and other people who don't.