Is the inquirer sinful for not being frank with her fiancé about the nature of her thin hair, fearing his reaction after marriage?

This fear is unfounded. You do not need to inform your fiancé about your sparse hair, as it is not a repellent flaw that hinders the complete enjoyment between spouses. If you wish, you may inform him to relieve yourself of fear and anxiety. If you wish, you may conceal it from him, and this is preferable if it might deter him from you.