Frightened talks only about DCs:
[...] You take a status penalty equal to this value to all your checks and DCs [...]
Compare it to Clumsy:
[...] You take a status penalty equal to the condition value to Dexterity-based checks and DCs, including AC, Reflex saves, ranged attack rolls, and skill checks [...]
The wording of Clumsy seems to imply that AC is just one of the DCs, but is not clear. I really hope it is, otherwise Frightened is not nearly as good as I thought.