It isn't the freezing but the bacteria, molds and yeast that get a chance to grow when the tomato is frostbitten that are more likely to be present in the garden than in the freezer.

Can you make the taste any better by adding sugar? Or by cooking with green peppers, they might overwhelm the bitterness.

I know sugar can cut a sour taste, but bitter is much harder to cover up. Maybe try ading a bit of lemon juice to see if that helps.

The spice note used in the flavor system may be selected from spices that are commonly known: preferably
ginger, clove, anise, cinnamon, nutmeg, or mixtures thereof. A preferred sweet note is vanillin, especially
ethyl vanillin. The fruity note may also be selected from among those known in the flavoring art, with
raspberry and lemon being preferred.

Another site said salt works well to cover bitter.
Here is the URL since I don't know how to list more than one. It's a pretty neat site.

http://www.exploratorium.edu/cooking/seasoning/kitchen/activity-quinine.html

But the primary reason is the change in pH frost causes. That makes it a safety issue rather than just one of flavor.

Dave

Research has found several conditions that can reduce the acidity of tomatoes. These include decay or damage caused by bruises, cracks, blossom end rot or Insects, and overripening. Tomatoes grown in the shade, ripened in shorter hours of daylight, or ripened off the vine tend to be lower in acidity than those ripened in direct sunlight on the vine. Also, tomatoes attached to dead vines at harvest are considerably less acidic than tomatoes harvested from healthy vines. Decayed and damaged tomatoes and those harvested from frost-killed or dead vines should not be home canned.

But it doesn't mention bitterness.