Some hair sprays are extremely flammable, yes. Most aerosol sprays are flammable, though, if only because a flammable gas is used to pressurize them. (though I know hairspray can be flammable after it's dried-on, too. It makes for some amusing hair accidents) I'd assume that the fixative is either acrylic or some sort of resin, so it may or may not be flammable. When dry it's likely to be less flammable than the paper you're spraying with it, though.
Of course, if your sketches are that close to an open flame that such a thing matters, I think you have other problems to worry about.