My best guess is that Derrick simply isn't funny when he's Britney?

This. From what she's shown us so far, Derrick doesn't seem very funny. I suspect that like a lot of the impersonator queens in the past, she has the look of her celebrity down, but just lip syncs their routines and does dance covers. Chad Michaels clearly has a lot of experience doing improv and comedy as Cher, and I don't think Derrick can say the same when doing her Britney - at the very least, she doesn't seem like she has a lot of improv experience.