If I add a 100 amp disconnect between my service entrance and my new 100 amp panel does the disconnect need to be fused?

Based on typical installations, and little details, I will say yes.

All new installations, at least in Georgia, require a service disconnect outside the house, usually in the meter box. This way, emergency personnel can turn the power off without going inside i.e. fire. There should be a 100 Amp breaker in the disconnect.

May I ask why you are doing this? The main breaker serves as the disconnect. Are you moving the panel away from the service entrance?

Not to waste any one's time, but if you install a main disconnect switch, the grounding system is affected and the panel would now require a four conductor feed and the removal of the main bonding jumper inside the panel.