Is it okay to have an ungrounded light fixture?
We live with legacy ungrounded fixtures all the time, usually without incident.  As I understand it, grounding is a backup system to give the high potential an easy path to ground (that is, a better path than YOU) if there is a short in the fixture. 

Basically, grounding is a connection between the metal body of a fixture to a continuous ground (wire or metal shielding) that returns to the service panel and an earth ground. You could create a ground on a fixture by installing a screw in the metal body of the unit that could then take the ground wire. However, there could be more than one metal component in the fixture needing a ground, so it is by no means a sure fix.

The main issue is the likelihood of a user touching something metal on the fixture that could be accidentally live.  If the fixture is out of reach, the lack of a ground is probably a low risk. If it is a pull chain light, use of a string pull rather than metal is a good idea.