Unfortunately, for some reason, there isn't an in-game setting for this. Instead you have to edit "WellingtonSystemSettings.ini" which by most default installations should reside here: C:\Program Files (x86)\Steam\SteamApps\common\Fable Anniversary\WellingtonGame\Config

I personally set this value to 1.5, and I wouldn't recommend going any higher than 2.0, though you're more than welcome to fiddle around with it at your discretion and comfort. Hopefully this helps anyone looking to run this game but has an issue with the camera's inherently low sensitivity.