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

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.

Take a paper towel and run it under water then ring it out. Place 3-4 seeds in the paper towel and place in a warm spot in your house. I usually use the top of the clothes dryer because it's used so often, but you could use a heating pad set on very low. Within a couple of days they should sprout if the seeds are still viable. Happy gardening!

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

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