If you're planting seeds from a pepper, is it okay if they were once frozen?

Seeds are routinely frozen to preserve them long term. As Jihil noted, they should be properly dried first.

And if the pepper in question is a hybrid variety, the plants, should the seeds germinate, won't come true.

Are you trying to plant seeds from a whole pepper that was once frozen, or to plant properly dried seeds that were frozen in a good container? If the entire pepper was frozen they probably won't germinate. If just the dried seed was frozen they might work - try putting a seed or two between moist paper towels for a few days to see if you get any germination action.